Evaluation of DRAINMOD for predicting water table heights in irrigated fields at the Jordan Valley

A detailed field experiment was carried out in the Jordan Valley, south of Lake Kinneret, Israel for evaluation of the water management model DRAINMOD. This field was chosen to represent the local agro-climate conditions of that zone. Banana crop was grown and was irrigated daily with about 3200 mm/year and 0.5 leaching fraction. Subsurface drainage system with 2.5 m drain depth and 160 m drain spacing existed in the field. The water table depth was measured with about 100 piezometers, in which most of them were observed weekly, and four were continuosly recording piezometers. Five identical drainage plots were selected, out of 10 existing, as replicates for the evaluation of DRAINMOD. Deviations in a range of 0.3–1.7 m between observed water table depth and that simulated by DRAINMOD were found in four out of the five replicates. A reasonable agreement was found only in one drainage plot out of the five tested. These findings contradict the world wide convention that DRAINMOD simulation is in a good agreement with observed field data. An additional study was therefore conducted to explore the reasons for these large deviations. Three reasons were suggested: (i) a strong side effect by the Jordan River, which flows some 350 m west to the test field; a very steep 4.6% gradient was found toward the Jordan River; (ii) presence of sandy permeable layers below the depth of the drains which magnifies the boundary condition effect of the Jordan River; (iii) a very significant component of deep and lateral seepage (more than 50% of the yearly irrigation plus rainfall). A combination of these three reasons was suggested as an explanation to the apparent large disagreement. It was therefore recommended not to use DRAINMOD or similar vertical flow models for simulation of water table depths in irrigated fields with subsurface drain pipe systems in the Jordan Valley.