CHARACTERIZING MICROTOPOGRAPHICAL EFFECTS ON LEVEL-BASIN IRRIGATION PERFORMANCE

Microtopography has long been recognized as one of the key variables i n level-basin irrigation performance, although little effort has been devoted to establish its relevance. In this work, experimental data ar e used to quantify the influence of microtopography on irrigation perf ormance. An irrigation evaluation was performed on a small level-basin (256 m(2)) LASER levelled to zero slope. Irrigation depth was gravime trically measured and estimated at the 49 nodes of a regular network. Data from the irrigation evaluation and a two-dimensional flat-bed mod el were used to estimate irrigation depth. Irrigation times, soil surf ace elevation and distance to the inlet were estimated at the same nod es, and a correlation matrix was computed. Results showed that soil su rface elevation was highly and significantly correlated with the times of advance (0.725**), recession (-0.815***) and opportunity (-0.852* *), and with the measured irrigation depth (-0.583***). Distribution uniformity using soil water measurements was 71.0%, Estimates from the irrigation evaluation and the two-dimensional model were 85.3% and 94 .9%, respectively, The irrigation evaluation procedure could explain 3 0**% of the measured variability in irrigation depth. A large part of the unexplained variance in measured irrigation depth seems to be due to the spatial variation of infiltration properties. Predictions by t he two-dimensional model were not significantly related to the measure d values. A simple method was devised to estimate microtopography-adju sted irrigation performance from the results of a Aat bed model and th e standard deviation of elevation. Microtopography can have an importa nt effect on level-basin irrigation performance. Models not considerin g this variable may incur large errors when simulating irrigation perf ormance.