Comparison of Locally Determined Crop Coefficients to FAO 56 (The Case of Ethiopia)

Abstract

Proper irrigation scheduling and effective water management are impossible without the exact estimation of crop coefficient values in agroecology and crop variety. There is a research gap in the relevance of locally determined crop coefficients compared to the previously determined crop coefficients by FAO56. This review paper aims to compare and evaluate the locally determined crop coefficients with the crop coefficients documented by FAO56. Although different crops are grown in Ethiopian agroecology, only some crops have locally determined crop coefficients. These crops are onion, haricot bean, barley, wheat, tomato, teff, sugarcane, sorghum, cabbage, pepper and sesame. Except for the native crop (teff) of Ethiopia, the locally determined crop coefficient values of all others are compared with the values documented by FAO56. Accordingly, the locally determined crop coefficient values are significantly different compared to the values documented by FAO56. Considering pepper crops as a sample, updated Kc values were 1.19, 1.31, and 1.01 at the initial, mid, and late growing stages; whereas 0.6, 1.05, and 0.9 at the initial, mid, and late growing stages were documented in FAO56 document. The difference that happened between the locally determined crop coefficients and values determined by FAO56 is due to differences in crop variety, agroclimate, and the methods followed for water application and estimation of crop evapotranspiration. In line with this, the use of crop coefficient documented in FAO56 impacts water management and crop productivity, particularly in water scarcity. As reviewed different locally determined crop coefficients for different crops from different research papers, the values oscillated on the crop coefficient determined from FAO56. These oscillations indicate that it causes loss of water and aeration problems due to over-application of irrigation water. On the other hand, moisture scarcity in the crop root zones happens due to little irrigation water application to the crop. Therefore, local determination of crop coefficients for different varieties of crops in different agroecological zones is crucial for water resource projects for irrigation. Further research is required for locally determined crop coefficients considering crop variety, agroecology, and other scientific approaches.

1. Introduction

Proper irrigation scheduling and irrigation water management are important for the sustainability of irrigated agriculture. Proper irrigation scheduling and effective water management will be impossible without the exact estimation of crop coefficient values in that agroecology [1]. Mismanagement of water has been among the major problems observed in many of the irrigated agricultural fields. This could be due to a lack of information on the water requirements of locally determined crop coefficients [2]. In precision agriculture, crop evapotranspiration plays a major role.

As crop evapotranspiration is the most challenging component in agricultural water management, accurate crop evapotranspiration estimation is required to understand the water balance, hydrological processes, climatic variations, and ecosystem processes [3]. Since irrigation water is insufficient for the total agricultural demand, precise crop water requirements are essential for accurate management and conservation of agricultural water [4]. Precise and accurate crop water demand assessment needs accurate crop coefficient values to estimate crop evapotranspiration.

The crop coefficient represents crop-specific water use and is essential for accurate estimation of the irrigation requirements of different crops in the command area. It serves as an aggregation of the physical and physiological differences between crops and the reference definition [5, 6]. It is a key parameter commonly required for the estimation of crop evapotranspiration because it considers the biological characteristics of crops, crop condition, soil type, and crop growing environment [7]. The crop coefficient (Kc) values for the same crop may vary from place to place based on factors such as irrigation regime, management practices, local climate, soil types, and other environmental factors [8, 9, 10].

The crop coefficient is the main input parameter for water resource planning, development projects, and irrigation scheduling [11]. Even if it is a very important parameter for the estimation of crop water evapotranspiration for the planning of water resource projects, crop coefficients for some of locally available crop varieties with detail crop growth length in different agroecologies are not known in Ethiopia [10]. Furthermore, locally determined Kc information is not available for many important crops in Ethiopia. The main issue of this review paper is raised with the question of “Do locally determined Kc values differ from values documented in FAO56?”. Furthermore, there is no documentation of Kc values of crops grown in Ethiopia.

Therefore, local-based crop coefficients are necessary; although there are published Kc values for different crops, these values are commonly used in places where local data are not available. Abebe et al. [7] recommended to studying local-based Kc for accurate estimation of water use under specific climatic conditions. Laike et al. [6] also recommended for local calibration of crop coefficients under a given climatic conditions as the values of Kc varied in spatially and season to season. Moreover, since a database of crop coefficients of different crops in varying agroclimatic regions is not available and Kc values given by FAO56 are being used for irrigation scheduling of different crops [12, 13], it is necessary to generate region-specific crop coefficients under the given climatic conditions. Therefore, this paper focused on the review of the significance of locally based determinations of crop coefficients that are grown in Ethiopia.

This paper attempted to review the crops that have had crop coefficients in Ethiopia only. The author tried to compare the locally determined crop coefficients with the values determined by the FAO 56 document. This paper does not include the different crop coefficients that are done out of Ethiopia, except used as evidence for scientific principles.

2. Review methodology

The organization of this review paper was based on earlier research. Google for published data and university repositories for unpublished research outputs were the sources of this information. The researchers are well acknowledged (Tables 1 and Table 2), and the data for this review paper came from secondary research outputs. Utilizing the information provided by the acknowledged researchers, graphs were created. The developed graphs were analyzed and interpreted using Microsoft Excel.

Table 1: Locally determined Kc values in Ethiopian agroecology

Crop		Crop coefficients at each growth stage				Location		Sources
Type	Variety	Ini.	Dev.	Mid.	Late	Name	Altitude (masl)
Onion	R. Bombay(M)	0.57	0.78	1.03	0.77	WARC	750	[7]
	R. Bombay(C)	0.49	0.9	1.01	0.79		750
	Red Bombay	0.61	0.86	1.02	0.8	MARC	1550	[5]
	Red Bombay	0.47	-	0.99	0.46	MARC	1550	[18]
	Unknown	0.67	0.93	1.24	0.95	Dangishta	NS	[26]
	Onion dry	0.7	-	1.05	0.75	Med.	NS	[8]
Haricot bean	Unknown	0.34	0.7	1.01	0.68	MARC	1550	[6]
	Dry bean	0.4	-	1.15	0.35	CC	NS	[8]
Barley	Unknown	0.6–0.8	0.6–1	1–1.05	0.3–0.4	Tigray	2130	[2]
	Unknown	0.3	-	1.15	0.25	CI	NS	[8]
Wheat	Kekeba	0.54	-	1.15	0.67	MARC	1550	[19]
	Unknown	0.57	0.92	1.19	0.7	MARC	1550	[20]
	Unknown	0.3	-	1.15	0.4	CI	NS	[8]
Tomato	Melka Shola	0.57	0.86	1.13	0.88	MARC	1550	[21]
Tomato (C)	Melka Salsa	0.63	0.84	1.15	0.84	WARC	750	[22]
Tomato (M)	Melka Salsa	0.64	0.97	1.22	0.89
	Unknown	0.6	-	1.15	0.7–0.9	Med.	NS	[8]
Teff	Unknown	0.8–1.0	0.95–1	0.95–1.1	0.4–0.5	Mekelle	2130	[2]
	Gemeches	0.46	0.88	1.02	0.57	MARC	1550	[23]
	Quncho	0.6	0.8	1.1	0.8	MARC	1550	[27]
	Quncho	0.79	0.98	1.23	0.74	AM	1203	[24]
	Tseday	0.79	0.94	1.21	0.75	AM	1203
Barley	Unknown	0.3	0.75	1.05–1.2	0.25–0.4	CI	NS	[8]
Millet	Unknown	0.3	0.7	1.05–1.2	0.3–0.4	Pakistan	NS
Sugarcane	NCO-334	0.42	0.93	1.26	1.05	MARC	1550	[25]
	Unknown	0.5	0.8	1.3	0.8	Tendaho	340–365	[28]
	Virgin	0.4	-	1.25	0.75	LL	LL	[8]
Sorghum	Unknown	0.45	0.83	1.18	0.78	MARC	1550	[29]
	Unknown	0.7	-	1.05	0.55	Med.	NS	[8]
Cabbage	Unknown	0.71	1.02	1.19	0.84	Robit	NS	[26]
	Unknown	0.7	-	1.05	0.95	CD	NS	[8]
Garlic	Unknown	0.65	0.82	1.07	0.82	Dangishta	NS	[26]
	Unknown	0.56	1.42	1.43	0.59	Robit Bata	1828	[30]
	Unknown	0.7	-	1	0.7	Med.	NS	[8]
Pepper	Unknown	1.19	1.28	1.31	1.01	Dangishta	Highland	[26]
	Unknown	0.6	-	1.05	0.9	Medit.	NS	[8]
Sesame	Unknown	0.23	0.66	0.97	0.34	Humera	NS	[31]
	Unknown	0.35	-	1.1	0.25	China	NS	[8]

AM: Arba Minch, CC: Continental Climates, CD: Californian Desert, CI: Central India, LL: Low Latitudes, MARC: Melkassa Agricultural Center, Med. : Mediterranean, NS: Not specified, WARC: Werer Agricultural Research, Center, M: Main season, C: Cool season

Table 2: Crop growing length for locally developed crop coefficients

Crop		Growing Length (days)				Method of ETc estimation	Method of irrigation	Source
Type	Variety	Ini.	Dev.	Mid.	Late
Onion	R.Bombay (M)	25	30	40	25	DL-SWB-Neuron	Overhead	[7]
	R. Bombay (C)	25	30	40	25	DL-SWB-Neuron	Overhead
	R. Bombay	NS	NS	NS	NS	DL-SWB-neutron	Overhead	[5]
	R. Bombay	NS	NS	NS	NS	DL-SWB-neutron	Surface	[18]
	Unknown	NS	NS	NS	NS	SWB-TDR	Surface	[26]
	Onion dry	20	45	20	10	Standard	Standard	[8]
Haricot bean	Unknown	20	30	40	16	DL, SWB-neutron	Surface	[6]
	Dry bean	20	30	40	20	Standard	Standard	[8]
Barley	Unknown	15	25	70	30	DL, SWB-TDR	Surface	[2]
	Unknown	15	25	50	30	Standard	Standard	[8]
Wheat	Kekeba	20	25	35	20	DL-SWB-TDR	N/A	[19]
	Unknown	20	30	35	20	N/A	N/A	[20]
	Unknown	15	25	50	30	Standard	Standard	[8]
Tomato	Melka Shola	14	25	36	29	DL -SWB-neuron	N/A	[21]
Tomato(C)	Melka Salsa	20	30	40	20	DY-SWB-Neuron	Overhead	[22]
Tomato (M)	Melka Salsa	20	30	40	20	DY-SWB-Neuron	Overhead
	Unknown	30	40	45	30	Standard	Standard	[8]
Teff	Unknown	14–16	18–21	33–35	12–16	DL, SWB-TDR	Surface	[2]
	Gemeches	NS	NS	NS	NS	DL-NW-SWB-neuron	N/A	[23]
	Quncho	10	10	20	10	DL-SWB-Neuron	Overhead	[27]
	Quncho	14–20	20–21	23–25	11–12	WL-SWB	Overhead	[24]
	Tseday	14–15	16–18	17–20	13–15	WL-SWB	Overhead
Barly	Unknown	15	25	50	30	Standard	Standard	[8]
Millet	Unknown	15	25	40	25	Standard	Standard
Sugarcane	NCO-334	70	86	160	112	DL-NW-SWB-neuron	N/A	[25]
	Un known	90	92	183	60	Unknown	BEF	[28]
	Virgin	35	60	190	120	Standard	Standard	[8]
Sorghum	Unknown	N/A	N/A	N/A	N/A	DL, SWB-neutron	N/A	[29]
	Unknown	20	35	40	30	Standard	Standard	[8]
Cabbage	Unknown	N/A	N/A	N/A	N/A	SWB-TDR	Surface	[26]
	Unknown	40	60	50	15	Standard	Standard	[8]
Garlic Garlic	Unknown	N/A	N/A	N/A	N/A	SWB-TDR	Surface	[26]
	Unknown	30	35	45	30	SWB-TDR	Surface	[30]
	Unknown	N/A	N/A	N/A	N/A	Standard	Standard	[8]
Pepper	Unknown	N/A	N/A	N/A	N/A	SWB-TDR	Surface	[26]
	Unknown	30	35	40	20	Standard	Standard	[8]
Sesame	Unknown	N/A	N/A	N/A	N/A	N/A	N/A	[31]
	Unknown	20	30	40	20	Standard	Standard	[8]

BEF: Block ended furrow, N/A: Not available

2.1 Common crops grown in Ethiopia

Different crop groups are cultivated in Ethiopia. Namely cereals, vegetables, oil seeds, root crops, and others. Cereals: Five major cereals (teff, wheat, maize, sorghum, corn, barley, millet, rice, and oats) are the core of Ethiopia’s agricultural productivity and food economy [14, 15]. Teff, wheat, and barley are primarily cool-weather crops cultivated at altitudes above 1500 meters. Barley is grown mostly between 2,000 and 3,500 meters. Sorghum, millet, and corn are cultivated mostly in warmer areas at lower altitudes along the country’s western, southwestern, and eastern peripheries. Corn is grown chiefly between elevations of 1,500 and 2,200 meters and requires large amounts of rainfall to ensure good harvests.

Vegetables: Common vegetable crops grown in Ethiopia are lettuce, head cabbage, Ethiopian cabbage, tomatoes, green peppers, red peppers, and Swiss chard [16]. Oilseed: The three major oilseed crops in Ethiopia are sesame, soybean, and niger seed [17]. Sesame is one of the oil seeds that grow at elevations from sea level to about 1,500 meters [14]. Root crops: Common root crops grown in Ethiopia are beetroot, carrot, onion, potatoes, Yom/boye, garlic, Taro/Godere, and sweet potatoes [16].

3. Locally determined crop coefficient values in Ethiopia

The common crops that are grown in Ethiopia are studied for their locally based crop coefficients at different times and locations. In line with this, locally based crop coefficient values of onion crops were studied at the Melkasa agricultural research center [5, 18] and at the Werer agricultural research center [7]. Similarly, the crop coefficient for haricot beans was determined at the Melkasa Agricultural Research Center [12]. Araya et al. [2] also studied the crop coefficient for barley in the Tigray region. Tezera et al. [19] and Gebre [20] also studied wheat crop coefficients at the Melkasa Agricultural Research Center. The crop coefficient for tomato crops at Melkasa Agricultural Research Center [21] and Werer Agricultural Research Center [22] was studied. From cereal crops, crop coefficients for teff crops were studied at Mekelle [2], Melkasa Agricultural Research Center [23], and Arba Minch [24]. Yadeta et al. [25] also studied the crop coefficient values of sugar cane at the Melkasa Agricultural Research Center.

Crop coefficients for different crops were documented in general conditions but may vary substantially from region to region with climate, cropping conditions, and crop variety. The user is strongly encouraged to obtain appropriate local information [8]. Therefore, the crop coefficient values for different crops studied in different climate conditions and crop varieties in Ethiopia are reviewed and organized as shown in Table 1. The other factor is to observe different values of Kc, which might be also the difference in crop growth length. As shown in Table 2, the crop growth lengths for the same crops were different. This variation of crop growth length may be due to different factors such as the agro-climatic zone of the crop grown area.

3.1 Vegetables (tomato, cabbage and pepper)

Tomato is one of the most economically important and widely grown vegetable crops under both rainfed and irrigation conditions in Ethiopia [32, 33]. Although this crop is widely cultivated using irrigation in Ethiopia, only two scholars have studied the local crop coefficient values for this crop [21, 22]. Figure 1(a) declares the graphical interpretation of crop coefficients for tomato crops, which is based on the collected data from the previously studies [8, 21, 22]. As shown in Table 1, the locally determined crop coefficient values for tomato crops are significantly different compared to those of the FAO56. The difference has also been observed between the two Ethiopian researchers. The difference is not only in the crop coefficient but also in crop growth length, which is one reason why the different Kc value is observed. As shown in Table 2, the total crop growing length for tomatoes is different for different varieties as well as growing seasons. Accordingly, the growing length for the tomato crop reported by Dirirsa et al. [21] was 104 days, which is less than the values reported by FAO (145 days). This growing length is also less than the 110 days reported by Abebe and Kebede [22]. The reasons for this difference are: (1) due to differences in crop varieties, which are Melka Shola for Dirirsa et al. [21] and Melka salsa for Abebe and Kebede [22]. Secondly, due to the difference in agroclimatic conditions between the two crop-growing areas. In line with this, Dirirsa et al. [21] studied the crop coefficient values for the Melka Shola variety in a midland agricultural area (1550 masl). Conversely, Abebe and Kebede [22] studied the local crop coefficient values for tomato crops in lowland areas (750 masl). The crop growing season also affects the crop coefficient, as reported by Abebe and Kebede [22], which is why different crop coefficient values were observed at the same crop variety and location in the area. The crop coefficient values for the main season of the Melka Salsa tomato variety were 0.64, 1.22, and 0.89 for the initial, mid, and late seasons, whereas 0.63, 1.15, and 0.84 were observed for the cool season at the initial, mid, and late seasons, respectively.

Figure 1: Comparison of Kc for tomato (a), cabbage (b) and pepper (c) MARC: Melkasa Agricultural Research Center, WARC: Werer Agricultural Research Center, M: Main Season, C: Cool season, FAO: Food, Agriculture and Organization

The locally determined Kc values of cabbage were 0.71, 1.19, and 0.84 at the initial, mid, and late stages [26] but the values of Kc documented in FAO were 0.7, 1.05, and 0.95 at the initial, mid, and late stages, respectively [8]. These Kc values of cabbage are significantly different compared to FAO values except for the initial stage (Table 1). As shown in Fig. 1 (b), the Kc values estimated by FAO are significantly lower than the locally determined Kc values of the cabbage crop at mid growing stage. Let’s say a researcher wants to estimate crop water requirements using the Kc values determined by FAO, and the output at the mid-growing stage becomes lower than the local crop water demand. This will cause water scarcity in the local or current crop root zone of the agricultural area. In line with this, the crop productivity of cabbage has not improved. Therefore, the locally determined Kc value for cabbage is essential for the estimation of crop water requirements.

Similar to the cabbage crop, the Kc values determined by local experts for the pepper crop are different compared to FAO values for all growing seasons (Fig. 1(c)). The locally determined Kc values for pepper were 1.19, 1.31, and 1.01 at the initial, mid, and late growing stages [26]. On the other hand, 0.6, 1.05, and 0.9 at the initial, mid, and late growing stages were reported by the FAO [8]. This difference may be caused by the crop variety or climate conditions of the two studied areas. There is no information about the variety of the pepper crop that has been studied both by local scholars and FAO experts. The local Kc value of pepper crops was studied in highland agricultural areas, but the FAO determined it in Mediterranean climate conditions. The difference in agroclimatic conditions is the main factor that affects Kc values [8].

Figure 2: Comparison of locally determined Kc for Barley (a), Wheat (b), Teff crop (c) and Sorghum (d) with FAO. AM: Arba Minch, M: Mekelle, MARC: Melkasa Agricultural Research Center, WARC: Werer Agricultural Research Center, FAO: Food, Agriculture and Organization

3.2 Cereals (barley, wheat, teff and sorghum)

There are several crops grouped in these cereal groups. From those crops, this paper reviewed only barley, wheat, teff, and sorghum which have locally determined crop coefficients. The comparison of locally determined crop coefficients of those crops with FAO crop coefficients is listed in Table 1 and Fig.2.

The locally determined crop coefficient for barley is graphically different from the values determined by FAO. Accordingly, the locally determined crop coefficients at the initial, mid, and late stages were 0.8, 1.0, and 0.3, respectively [2]. Whereas the crop coefficient values reported by FAO at the initial, mid, and late growing stages of barley were 0.3, 1.15, and 0.25, respectively [8]. In the figurative presentation of the barley crop coefficient determined by local scholars and FAO, a significant difference is observed (Fig.2(a)). This difference occurred due to variations in the crop growth length at mid growth stage [2]. The recorded crop growth length by Ethiopian researcher at mid stage increased by 20 days compared to FAO (Table 2). The other factors may be due to the varied barley crop variety, the agroclimatic conditions of the crop grown area and the crop grown length. The crop variety for barley is not specifically known. The local crop coefficient for barley was determined for midland agroclimatic conditions. Furthermore, the crop-grown lengths for the two sides are different. In line with this, the crop grown length for the locally determined crop coefficient is 140 days, whereas 120 days were observed for the FAO report (Table 2).

On the other hand, the locally determined crop coefficients for wheat crops are different compared to the FAO report, particularly at the initial and late crop growth stages (Fig.2(b)). The locally determined crop coefficients for the Kekeba wheat variety at the initial, mid, and late growth stages were 0.54, 1.15, and 0.67, respectively [19]. Similarly, for the unknown wheat variety, the locally determined crop coefficients at initial, mid, and late-grown stages were 0.57, 1.19, and 0.7, respectively [20]. These two scholars studied their crop coefficient values for wheat crops at the same climate condition or location, that is, Melkasa Agricultural Research Center. The agroecology of the study area is midland. Therefore, the result of the locally determined crop coefficient for the wheat crop is nearly similar. However, compared to the FAO report, the locally determined crop coefficient value was significantly different. Therefore, the difference is due to an agroecological difference between the locally determined crop coefficient and the FAO report. Furthermore, the crop-grown length for wheat crops is different, which has a significant impact. In line with this, the crop-grown length for locally determined crop coefficients was 100 days [19], 105 days [20], and 120 days [8].

The teff crop is well known by Ethiopians and is used as row material for the preparation of injera (local name). FAO has not studied the crop coefficient value for this crop. Water resources designers used the crop coefficient values of barley and millet instead of teff. However, the crop coefficient is significantly different from the assumed crop coefficient values of barley and millet. As shown in Fig.2(c), the crop coefficient of barley at the initial and late growth stages is significantly lower compared to the locally determined crop coefficient values of teff. It is difficult to take the common crop coefficient for different crops, although the crop coefficient varies on the crop variety of the same crop. The crop coefficient values for different varieties of teff crop are shown in Table 1. As seen in Table 1, the crop coefficient values of those varieties are significantly different. In line with this, Hordofa et al. [23] studied the crop coefficient values of the Gemeches teff variety at the midland agroclimatic zone (1,550 masl) of Melkassa Agricultural Research Center. They observed values of 0.46, 1.02, and 0.57 at the initial, mid, and late-grown stages, respectively. Yihun [27] also studied the crop coefficient values in the same agroclimatic zone, but the crop variety is different, which is the Quncho teff variety. In line with this, the observed crop coefficients are different, which are 0.6, 1.1, and 0.8 at the initial, mid, and late growth stages, respectively. However, the crop coefficient for the Quncho teff variety was determined at another agroecological zone, which is at Arba Minch, and different values were observed. The reported values of the crop coefficient are 0.79, 1.23, and 0.79 at the initial, mid, and late growth stages, respectively [24]. On the other hand, Araya et al. [2] determined the crop coefficient at Mekelle (highlands) and observed different values than others. The observed crop coefficient values at the highland were 0.8–1, 0.95–1.1, and 0.4–0.5 at the initial, mid, and late crop growth stages [2]. This indicated that crop variety and the agroecology of the crop-grown area significantly affect the crop coefficient. According to Zheng et al. [34], the crop coefficients for the two varieties of tea were different, with reported Kc values of 0.71 (range of 0.43–1.02) for BY1 and 0.84 (range of 0.48–1.22) for the LJ43 tea variety. Similarly, El-Noemani et al. [35] support the idea with their result. They reported that Kc ranged between (0.63–0.64) for initial, (0.87–0.82) for development, (0.99–1.09) for midseason, and (0.80-0.95) for harvesting stage in the case of the Bronco bean variety, while it ranged between (0.59–0.61) for initial, (0.78–0.98) for development, (1.07–1.19) for midseason, and (0.73–0.88) for harvesting stage in the case of the Contender bean variety. In line with this, Allen et al. [8] collected the Kc values for several crop varieties in different agroecologies. They report that the crop coefficients for the same crop variety have different values in different agroclimatic conditions. Furthermore, Guerra et al. [36] also reported different values of Kc for the same crop, mainly because of different weather conditions. The results showed that due to changes in mean daily minimum relative humidity and mean daily wind speed, the adjusted crop coefficients varied and, due to climate changes, changed for the selected Boro rice varieties consistently, both temporally and regionally [37].

Sorghum is also one of the cereal crops grown in Ethiopia and has a locally determined crop coefficient. The crop coefficient values of sorghum are significantly different compared to the FAO report (Table 1). The locally determined crop coefficient value at initial growth is lower compared to the FAO report but not at mid and late-growth stages (Fig.2(d)). The locally determined crop coefficients for sorghum were 0.45, 1.18, and 0.78 at the initial, mid, and late growth stages, respectively [29].

3.3 Root crops (onion)

Similarly, the onion crop is one of the most important crops in Ethiopia, and it is used to make wot (the local name). The crop coefficient for this crop is determined at different research centers, as shown in Fig.3 (a). This figurative interpretation was developed using the data from previous researchers in Ethiopia.

As shown in Fig. 3(a), the Kc value determined in Ethiopia and FAO is significantly different for the onion crop. The Kc values determined by FAO are higher than the locally determined Kc values, except in the late stages. The difference occurred not only compared to FAO but also in the same place at MARC with the same onion variety (red Bombay), as studied by Dirirsa et al. [5] and Bossie et al. [18]. This difference might be expected from the methods of irrigation water application. Dirirsa et al. [5] used bucket watering (an overhead irrigation system). It is very important to prevent water loss through surface runoff and deep percolation. Contrarily, Bossie et al. [18] use surface irrigation (a furrow irrigation system) to apply the required irrigation water. The performance of the two irrigation systems is significantly different. In line with this, the amount of crop evapotranspiration is affected by the irrigation methods, and consequently, crop coefficients are also affected by the irrigation methods. Furthermore, the crop variety is the same as that studied at WARC, but the observed Kc values at different seasons (main season and cool season) were different [7]. This indicates that the climate conditions of the two growing seasons are different. The climate conditions of the crop growing season are the main factor that affects crop coefficient values. In general, the difference may be due to the agroclimatic conditions of that area, soil type (water holding capacity) and crop variety. Accordingly, crop coefficients are affected by a variety of crops and climatic conditions [5, 9]. Accordingly, Guerra et al. [36] reported different values of Kc for the same crop, mainly because of different weather conditions. Furthermore, Rahman et al. [38] concluded that climate variability has a significant impact on crop coefficient values. The crop-grown areas of those varieties were different.

The other root crop, in addition to onions, is garlic. Garlic is one of the important crops for traditional medicinal plants. Because of its importance, there is no consideration for further investigation of this important crop. The crop coefficient is one of the important coefficients for further investigation of this crop to estimate accurate crop water requirements. As shown in Fig. 3(b), the crop coefficient for the garlic crop determined by FAO is lower than the locally determined crop coefficients at the mid-growth stage but not at the initial crop-growth stage. Unfortunately, the locally determined crop coefficient [30] at the mid-growth stage has a significant difference from the FAO document. This water has a significant impact on crop water determination and water resource development projects.

Figure 3: Comparison of locally determined Kc for onion crop (a) & garlic crop (b). MARC: Melkasa Agricultural Research Center, WARC: Werer Agricultural Research Center, FAO: Food, Agriculture and Organization, Di: Dirirsa et al. 2015, Bo: Bossie et al. 2009, Yim: Yimam et al. 2020, Get: Getachew, 2020

3.4 Oil seed (sesame crop)

Sesame is the only oil seed crop that has a local crop coefficient in Ethiopia, and a raw material for oil preparation. In the current Ethiopian context, oil industries are installed in the Amhara region. However, there is input scarcity for all oil industries. The main problem with this is the lack of annual productivity in sesame. Because this crop is cultivated using rain-fed agriculture. Therefore, this crop’s productivity should be improved using irrigation. To use irrigation, this locally developed crop coefficient is very important. As compared to the values of the crop coefficient developed by FAO, they are significantly different (Fig. 4). This variation may be due to the variety of sesame and the agroecology of the crop-growing area.

Figure 4: Comparison of locally determined Kc for Sesame crop to FAO value

3.5 Sugarcane

As shown in Fig.5, the result of the local crop coefficient for sugarcane is different from the crop coefficients documented by FAO. The maximum crop coefficients at the initial and mid-stage were observed for unknown varieties [28], and at the late stage, they were observed for the NCO-334 variety (Fig.5). The Kc values documented by FAO are the minimum values, except at the mid-crop growth stage. At this stage, unfortunately, the Kc values for the NCO-334 sugarcane variety [25] are the same as the values reported by the FAO [8]. In addition to crop variety, the agroecological zones of the two crop-growing areas are different. In line with this, Yadeta et al. [25] conducted their research at an altitude of 1550 masl, whereas [28] varied at an altitude of 340–365 masl Furthermore, the growing lengths for all varieties are significantly different. This might be one of the significant factors in observing the different Kc values in addition to crop varieties and agroecological zones of the crop-grown area.

Figure 5: Comparison of locally determined Kc for sugarcane growth in Ethiopia to FAO

3.6 Legume (haricot bean)

Although several crops are grouped as legumes and grown in Ethiopian agroecology, only haricot beans have updated crop coefficient values in Ethiopia. The updated crop coefficient values of haricot beans show a significant difference compared to the FAO results (Table 1).

Figure 6: Comparison of locally determined Kc for Haricot bean to FAO

As shown in Fig. 6, the FAO Kc value for haricot bean was overestimated compared to the locally determined Kc value, except in the late stage. This overestimation of crop coefficients directly contributes to the overestimation of crop water requirements. This is already one cause of irrigation water loss. The consequence of the overestimated crop water requirement is over-irrigation, which results in the development of waterlogging or water ponding in the agricultural area. Therefore, the use of locally determined Kc for haricot beans is more appreciated for managing irrigation water and improving crop productivity. Some crops are sensitive to excess water, which makes them unproductive. It is important to estimate crop coefficients more accurately because crops like cabbage are unproductive because of over-irrigation caused by exaggerated crop coefficients [39]. Over-irrigation consumes time, promotes the leaching of nitrogen and other micronutrients, increases energy demand for pumping, and results in water loss [40, 41, 42].

3.7 Crop coefficients determined only in Ethiopia

Most common crops grown in Ethiopia have no crop coefficient values in the FAO document [8]. Due to the lack of this important information, the accurate determination of irrigation water demand for the design of irrigation projects was difficult. In this case, designers substituted the missing information with similar crops. For instance, since there was no field research conducted to determine the Kc of teff, irrigation planners and managers, as well as researchers, used the Kc values of barley and millet [27]. However, the Kc values of barley and millet are significantly different for the initial and late-growth stages [27]. Araya et al. [2] also approved that the mid-season Kc values of teff were slightly lower than the values for wheat and barley (common cereal crops) (Table 1). Therefore, the only cereal crop that has a Kc value in Ethiopia is teff. Any water resource designer uses the Kc values of teff crops determined by the above scholars.

4. Conclusion

Crop coefficient is an essential parameter for irrigation projects, and it is affected by crop varieties and climate conditions. Based on the review of the different research works, the crop coefficients are different from those previously determined by the FAO 56 document. Not only compared to FAO 56, but the crop coefficient values studied in the same area are also different. The impact here is due to the different crop varieties and the methods followed. In line with this, different varieties of crops have different characteristics. As an example, some crops are harvested in a short period, but others take a long period. Furthermore, as reviewed in different research papers, they followed different methods of irrigation water application and crop evapotranspiration estimations. This might also be the cause of the different values of the crop coefficient observed. In addition, the crop coefficient is affected by the agroecology or climate conditions of the crop-growing area. Therefore, water resources and irrigation project planners, designers, implementers, users, and the scientific communities are recommended to use these updated locally determined Kc values based on the agroecological behavior of the project area.

For future directions, as indicated in this review paper, there is a lack of research related to the determination of local crop coefficients for various crops in various agro-climatic zones of Ethiopia. Therefore, researchers try to address these research gaps for the economic development of Ethiopia and the globe through modernized irrigation.

References

• [1] Mila AJ, Akanda AR, Biswas SK and MH (2016) Ali crop co-efficient values of Sunflower for different growth stages by lysimeter study. British J. of Envir. & Clim., 6(1): 53–63.
• [2] Araya A, Solomon H, Mitiku H, Sisay F and Tadesse D (2011) Determination of local barley (Hordeum Vulgare) crop coefficient and comparative assessment of water productivity for crops grown under the present pond water in Tigray, Northern Ethiopia. CNCS, Mekelle Univer., 3(1): 65–79.
• [3] Wanniarachchi S and Sarukkalige R (2022) A review on evapotranspiration estimation in agricultural water management: Past, present, and future. Hydrolo., 9: 123.
• [4] Koech R and Langat P (2018) Improving irrigation water use efficiency: A review of advances, challenges and opportunities in the Australian context. Water, 10: 1771.
• [5] Dirirsa G, Hordofa T and D Bekele (2015) Water requirement and crop coefficient of onion (Red Bombay) in the Central Rift Valley of Ethiopia. Inter. J. of Recent Rese. in Life Sci., 2(1): 1–6.
• [6] Laike S, Tilahun K and Hordofa T (2006) Crop coefficient of Haricot Bean at Melkassa, Central Rift Valley of Ethiopia. J. of Agric. and Rur. Develo. in the Trop. and Subtrop., 107(1): 33–40.
• [7] Abebe N, Kebede E, Derese Y, Robi F and Nanesa K (2021) Determination crop coefficients and water requirement of onion by using lysimeter at Werer, Middle Awash Valley of Ethiopia. J.R.S.A.S., 7(3): 14–21.
• [8] Allen R, Pereira L, Raes D and Smith M (1998) Crop evapotranspiration; guidelines for computing crop water requirements. Food and Agricultural Organization, Irrigation and Drainage Paper No. 56, Rome, Italy.
• [9] Abedinpour M (2015) Evaluation of growth-stage-specific crop coefficients of maize using weighing lysimeter. Soil and Water Rese., 10(2): 99–104.
• [10] Pereira LS, Paredes P, Hunsaker DJ, Lopez-Urrea R and Shad ZM (2021) Review: Standard single and basal crop coefficients for field crops. Updates and advances to the FAO56 crop water requirements method. Agric. Water Manage., 243: 1–33.
• [11] Kenjabaev S, Frede HG, Begmatov I, Isaev S and Matyakubov B (2020) Determination of actual crop evapotranspiration (ETc) and dual crop coefficients (Kc) for cotton, wheat and maize in Fergana Valley: Integration of the Fao-56 Approach and Budget. J. of Crit. Review, 7(5): 340–349.
• [12] Hussain F, Shahid MA, Majeed MD, Ali S and Zamir MS (2023) Estimation of the crop water requirements and crop coefficients of multiple crops in a semi-arid region by using lysimeters. Envir. Sci. Proceed., 25: 101.
• [13] Gupta A, Sarangi A and Singh DK (2017) Estimation of crop coefficients and water productivity of mustard (Brassica juncea) under semi-arid conditions. Curr. Sci., 113(2): 264–271.
• [14] Taffesse AS, Dorosh PA and Gemessa SA (2012) Crop production in Ethiopia: Regional patterns and trends. In Food and Agriculture in Ethiopia: Progress and Policy Challenges, ed. Paul A. Dorosh and Shahidur Rashid. Chapter 3 Philadelphia, PA: University of Pennsylvania Press, 53–83.
• [15] Dessie A (2018) Cereal crops research achievements and challenges in Ethiopia. Intet. J. of Rese. Stud. in Agric. Sci., 2018, 4(6): 23–29.
• [16] Tadesse B and Bekele K (2022) Scope of vegetable and root crops value chain in Ethiopia. J. of Agric. and Food Rese., 10: 100402.
• [17] Bickford R (2020) Ethiopia oilseeds report annual. Report number: ET2020-0001, USDA.
• [18] Bossie K, Tilahun K and Hordofa T (2009) Crop coefficient and evapotranspiration of onion at Awash Melkassa, Central Rift Valley of Ethiopia. Irrig. and Drain. Sys., 23(1): 1–10.
• [19] Tezera K, Dirirsa G and Hordofa T (2019) Determination of wheat (Triticum Aestivum L) seasonal water demand and crop coefficient for effective irrigation water planning and management in semi-arid, Central Rift Valley of Ethiopia. Inter. J. of Enviro. Sci. & Nat. Reso., 21(1): 556054.
• [20] Gebre NN (2021) Determination of crop water requirement and crop coefficient at different growth stages of lowland bread wheat (Triticuma Estivum L.) at Melkassa, Oromia, Ethiopia. M.Sc. Thesis, Haramaya University, Haramaya, Ethiopia.
• [21] Dirirsa G, Hordofa T and Bekele D (2017) Crop coefficient and water requirement of tomato (Melka Shola Variety) at Melkassa, Central Rift Valley of Ethiopia. Acad. Rese. J. of Agric. Sci. and Rese., 5(5): 336–340.
• [22] Abebe N and Kebede E (2020) Determination of crop coefficients and water requirement of tomato at Werer, Middle Awash Valley of Ethiopia. Inter. J. of Agric. and Biosci., 9(4): 152–155.
• [23] Hordofa T, Dirirsa G, Wondimu T, Tesfaye A and Tezera K (2020) Crop water requirement and crop coefficient of Tef (Eragrostis tef) in Central Rift Valley of Ethiopia. J. of Nat. Sci. Rese., 11(15): 34–39.
• [24] Asres LA and Derebe MA (2022) Determination of crop coefficients and yield response factors for different varieties of Teff (Eragrostis Tef). Thematic research, Arba Minch University, Arba Minch, Ethiopia.
• [25] Yadeta B, Ayana M, Yitayew M and Hordofa T (2021) Determination of water requirement and crop coefficient for sugarcane using lysimeter Experiment under semiarid climatic conditions of Ethiopia. J. of Irrig. and Drain. Engi., 147(11): 04021051.
• [26] Yimam AY, Assefa TT, Adane NE, Tilahun SA, Jha MK and Reyes MR (2020) Experimental evaluation for the impacts of conservation agriculture with drip irrigation on crop coefficient and soil properties in the sub-humid Ethiopian highlands. Water, 12: 1–20.
• [27] Yihun YM, Haile AM, Schultz B and Erkossa T (2013) Crop water productivity of irrigated teff in a water stressed region. Water Reso. Manag., 27: 3115–3125.
• [28] Shonka TG and Gullie G (2017) Determining the optimum decision variables and rescheduling the irrigation system of Tendaho Sugar Estate, Ethiopia. Irrig. and Drain. Syst. Engin., 6(3): 1–6.
• [29] Shenkut A, Tesfaye K and Abegaz F (2013) Determination of water requirement and crop coefficient for sorghum (Sorghum bicolor L.) at Melkassa, Ethiopia. Sci. Techno. and Arts Rese. J., 2(3): 16–24.
• [30] Getachew T (2020) Development of crop coefficient and evaluation of partial nutrient balance for garlic (Allium sativum) and inter cropped Napier grass under dry period irrigation in Robit Bata watershed. DSpace Institution.
• [31] Berhane A (2020) Crop coefficient, yield response factor and water use efficiency of sesame under full and deficit irrigation conditions in Humera, Western Tigray. Agrisummit 2020, 14th International Conference on Agriculture & Plant Science, Webinar, June 22–23.
• [32] MoA (Ministry of Agriculture) (2013) Crop variety register, Issue No. 16. Addis Ababa Ethiopia.
• [33] AVRDC (Asian Vegetable Research and Development Center) (2014) Scoping study on vegetables seed systems and Policy in Ethiopia, report.
• [34] Zheng S. Ni K, Ji L, Zhao C, Chai H, Yi X, He W and Ruan J (2021) Estimation of evapotranspiration and crop coefficient of rain-fed tea plants under a subtropical climate. Agron., 11: 1–13.
• [35] El-Noemani AA, Aboamera MAH and Dewedar OM (2015) Determination of crop coefficient for bean (Phaseolus vulgaris L.) plants under drip irrigation system. Inter. J. of ChemTech Rese., 8(12): 203–214.
• [36] Guerra E, Snyder RL and Ventura F (2011) Crop coefficients to adapt for climate change impact on evapotranspiration. Italian J. of Agro. meteoro., 14(2): 15–16.
• [37] Rahman MA, Karim N, Kadir MN and Naher T (2015) Impacts of climate change on crop coefficient and reference crop evapotranspiration of boro rice in north-east hydrological region of Bangladesh. Conference: Proceedings of the 5th International Conference on Water & Flood Management (ICWFM).
• [38] Rahman MA, Ali M, Mojid MA, Anjum N, Haq ME, Kainose A and Dissanayaka CR (2022) Crop coefficient, reference crop evapotranspiration and water demand of dry season boro rice as affected by climate variability: a case study at the north–east Bangladesh. Irrig. and Drain., 71(3): 1–18.
• [39] Seidel SJ, Werisch S, Schütze N and Laber H (2017) Impact of irrigation on plant growth and development of white cabbage. Agric. Water Manag., 187: 99–111.
• [40] Irmak S (2008) Plant growth and yield as affected by wet soil conditions due to flooding or over irrigation. University of Nebraska–Lincoln.
• [41] Sun W, Hamani AKM, Si Z, Abubakar SA, Liang Y, Liu K and Gao Y (2022) Effects of timing in irrigation and fertilization on soil NO₃^--N distribution, grain yield and water-nitrogen use efficiency of drip-fertigated winter wheat in the north China Plain. Water, 14: 1780.
• [42] Wang Y, Chen J, Sun Y, Jiao Y, Yang Y, Yuan X, Lærke PE, Wu Q and Chi D (2023) Zeolite reduces N leaching and runoff loss while increasing rice yields under alternate wetting and drying irrigation regime. Agric. Water Manag., 277: 108130.