An efficient methodology for estimating irrigation return flow coefficients of irrigated crops at watershed and seasonal scale

Irrigation return flow coefficients, i.e. the ratio between the quantity of water returned from the cultivated area to the groundwater system and the amount of abstraction, vary by more than 50% for rice cultivation using standing water irrigation to 0% in the case of drip irrigation technique. This component of the groundwater budget plays an important role, particularly in intensively irrigated areas. Thus, to avoid any inaccurate aquifer budgeting, modelling and consequently any erroneous watershed management, this component needs to be accurately assessed for a particular time-step (e.g. weekly, seasonally) onto the studied area. The present paper proposes a cost-effective and useful methodology for assessing irrigation return flow coefficients (C(f) = irrigation return flow/pumping flow) based on (i) basic crops field survey and meteorological data and (ii) the use of a simple hydraulic model that combines both water balance technique and unsaturated/saturated flow theory. An attempt to estimate the uncertainty of irrigation return flow coefficient estimates based on the uncertainty introduced by the pumping and the natural spatial variability of the soil characteristics is also proposed. Results have been compared to real field conditions and allow us to (i) estimate the uncertainty and (ii) validate and demonstrate the robustness of the applied methodology. The proposed methodology allows relatively good estimates of the irrigation return flow coefficients at watershed and seasonal scale. The irrigation return flow coefficients are calculated as: 51 +/- 8% in rainy season (Kharif) and 48 +/- 4% in summer (Rabi) for rice; 26 +/- 11% in rainy season and 24 +/- 4% in summer for vegetables; 13 +/- 8% in rainy season and 11 +/- 3% in summer for flowers. These results were found to be consistent with the existing literature. Copyright (C) 2007 John Wiley & Sons, Ltd.