ON WATER UTILIZATION OF IRRIGATION RESERVOIRS (I)

Abstract

In Japan, shortage of water is a very grave problem. It is very important to develop water resources and to use water more effectively. A field investigation on the usage of water from reservoirs for the irrigation of paddy fields has been made since April 1960. As the study reservoir, the MannR Reservoir (located in Kagawa Prefecture in southwestern Japan) having a water area of 1.385km², 1540×10⁴m³ of water planned pondage, and 19.7km in circumference was selected.
From the reservoir, 45.37 km2 of paddy fields is irrigated throughout 12 main irrigation channels, 54.2k in total length, during the entire irrigation period (from June to September). The drainage basin (96.02km²) into the Mannô Reservoir consists of two kind of basins: an indirect basin containing 85.71% of the total area, and a direct basin containing 14.29% of the area.
The following has been observed and calculated every day: 1) the water level of the reservoir, 2) the water pondage of the reservoir, 3) the water volume used for the irrigation from the reservoir, 4) the water volume of inflow into the reservoir from the indirect basin, and 5) the precipitation on the basin and the irrigation area. The following are some major results obtained:
1) During the nine years from 1960 to 1968, the water efficiency ratio of the reservoir, m, changed from 0.39 to 1.15. In the formula m=Q_T/Q_E, QT is the irrigation water used during an irrigation period and Q_E is the planned effective water pondage of the MannR Reservoir (=1540×10⁴m³). The value of _??_ (average of m from 1960 to 1963) was 0.77. During the nine years, only one year was with m_??_1.
2) The water distribution system of the MannR Reservoir has about 50 small sub-reservoirs connected by the 12 main channels. Water conveyed from the Mannô Reservoir by these main channels flows into these sub-reservoirs, and then it is distributed into every paddy field.
These sub-reservoirs are used several times in a year, because they are easily filled with rain and are easily emptied when water is used.
Therefore, water is poured into these sub-reservoirs from the MannR Reservoir only one or two times in a year. The water in the reservoir is reserved for usage during shortages of water. This is one of the reasons why the mean value of m of the Mann J Reservoir is as small as 0.77 for the nine years.
3) The relationship between m and P_yK (yearly precipitation at Kanakuragawa in the irrigation area, in mm) was expressed in the following equation, where r is the coefficient of correlation:
m=1.947-0.0009P_yK, r=-0.942
4) The relationship between the volume of irrigation water used during an irrigation period, Q_T (10⁷m³) and P_yK (10³mm) is shown by the following equation:
Q_T=3.123-1.432P_yK, r=-0.922
5) The value of Q_O/Q_E (Q_O is the water volume of the reservoir at the beginning of an irrigation period) varies from 0.99 to 1.01. Therefore, the reservoir is full of water at the beginning of every irrigation period.
The value of q_i/Q_E (q_i is the water volume flowed into the reservoir from the drainage basin during an irrigation period) varies from 0.25 to 0.49 and the average was 0.36.