Modeling water balance components and irrigation efficiencies in relation to water requirements for double-cropping systems

This paper describes the estimation of total water requirements as well as reservoir releases required to replenish the water deficit for double cropping of rice. A water balance equation is derived and the performance of a project is analyzed. The water balance components were modeled without calibration, and compared with measured data whenever possible. The potential evapotranspiration ETp were modeled for monthly time series using Penman-Monteith model [Monteith, J.L.,1965. Symp. Sec. Exp. Biol. 19, 205-234]. To estimate surface runoff, the proportion of elementary areas that are saturated were described by a spatial distribution function, considering the average total existing water (TEW) of both cropping seasons from 1991 to 1997. The monthly rainfalls (1971-1997) of 53 uniformly distributed rainfall stations were averaged and the rainfall considered uniform over the whole area. The effective meteorological input M-c was considered positive to produce runoff. The variation of the soil suction head and hydraulic conductivity with moisture content, studied by Brooks and Corey [Brooks, R.H., Corey, A.T., 1964. Hydraulic properties of porous media. Hydrology Paper No. 3, Colorado State University, Fort Collins, CO] was applied in the Green-Ampt model [Green, W.H., Ampt, G.A., 1911. J. Agric. Sci. 4 (1), 1-24] to estimate infiltration. The performance of a farm irrigation system is, in general, evaluated by the efficiency of the system. The nonlinear Muskingum-Cunge method [Cunge, J.A., 1969. J. Hydraulics Res.,Int. Assoc. Hydraulics Res. 7 (2), 205-230] was applied as a distributed channel routing technique to calculate the conveyance losses from the study reservoir (Pedu Reservoir) to the end of the main canal, a total reach of about 108 km. The mean conveyance efficiency from the Pedu dam to the Pelubang barrage (67 km reach) was found to be 66%, while the same from the Pedu dam to the end of the main canals was found to be 59%. Using Central Canal Right Bank Drain (CCRBD) A1 block data as reported by Batumalai and Nassir [Batumalai, R., Nassir, M.A.,1986. Effective water utilization with on farm terminal infrastructure. Northern Regional Water Seminar, Alor Setar, Malaysia], the distribution efficiency was determined to be 94%. The water use efficiency was estimated using MADA's field water supply data. The overall project efficiency for the main and off-seasons were calculated separately and found to be 18 and 32%. respectively. The total water requirements for rice in the main and off-seasons were determined and the amounts of water needed to release from the reservoir to meet the crop water deficit for double cropping were estimated to be 1385 million cubic meters (MCM). (C) 2000 Elsevier Science B.V. All rights reserved.