Water and salt management of irrigated open fields using drainage water of an aquaponics

Abstract

An aquaponics which the water is repeatedly used between aquaculture and hydroponics facilities, is one of food production technologies effectively using water resource. Recently, we developed a new type of aquaponics combined with open culture in arid regions and conducted its feasibility studies in a pilot plant in Mexico. Since the drainage water discharged from the hydroponics in our system is finally used in open fields as irrigation water, it may be extremely a water-saving system. It has, however, potentially risk that accumulated excess salts in the open field. Therefore, we conducted open culture experiments in a pilot plant for monitoring the water and salt movements in soil of the open field in Mexico and estimated the salt accumulation risk through some numerical experiments. In the open culture experiments, irrigation was carried out once a day for about 40 minutes by a farmer who managed the test field. The amount of irrigation at one time was around 18 mm/d. Electric conductivity of irrigation water (EC_w) was around 4 dS/m. Soil water and salinity were monitored by capacitance sensors at the point of a 3-cm distance from a drip tube and a 12-cm deep from soil surface. Although the field experiments had been conducted for 120 days, there were no remarkable increases of soil water and salinity at the monitoring point. HYDRUS 2D/3D were used for numerical experiments. The simulation conditions were that the amounts of irrigation at one time were 18 mm/d and 6 mm/d and irrigated once a day the water of 2, 4 and 8 dS/m in EC. We had run the simulation model under the condition that the same climate with the potential evapotranspiration of 6.4 mm/d continued for 30 days. In this simulation, no remarkable salt accumulation in the root zone was estimated with the current irrigation condition (18 mm/d and 4 dS/m of EC), but the percolation loss toward lower layers were greater than water-saving irrigation condition. Although under the water-saving irrigation condition, the moisture condition of root zone was adequate for crops, it proceeded salt accumulation at the edge of a wet zone which formed by the water dripped from the tube. However, the water-saving irrigation can leave much amount of water than the ordinary irrigation. We might be able to leachate the salts from root zone using the surplus water. These results showed that the drainage water of the aquaponics can be used as irrigation water without salt accumulation by the adequate irrigation control for open fields in drylands.