Long-term water balances in La Violada irrigation district (Spain): I. Sequential assessment and minimization of closing errors

Abstract

Long-term analysis of hydrologic series in irrigated areas allows identifying the main waterbalance components, minimizing closingerrors and assessing changes in the hydrologic regime. The main water inputs [irrigation (I) and precipitation (P)] and outputs [outflow (Q) and potential (ETc) crop evapotranspiration] in the 4000-ha LaVioladairrigationdistrict (VID) (Ebro River Basin, Spain) were measured or estimated from 1995 to 2008. A first-step, simplified waterbalance assuming steady state conditions (with errorɛ = I + P − Q − ETc) showed that inputs were much lower than outputs in all years (average ɛ = −577 mm yr−1 or −33% closingerror). A second-step, improved waterbalance with the inclusion of other inputs (municipal waste waters, canal releases and lateral surface runoff) and the estimation of crop's actual evapotranspiration (ETa) through a daily soil waterbalance reduced the average closingerror to −13%. Since errors were always higher during the irrigated periods, when canals are full of water, a third-step, final waterbalance considered canal seepage (CS) as an additional input. The change in water storage in the system (ΔW) was also included in this step. CS and ΔW were estimated through a monthly soil–aquifer waterbalance, showing that CS was a significant component in VID. With the inclusion of CS and ΔW in the waterbalance equation, the 1998–2008 annual closingerrors were within ±10% of total water outputs. This long-term, sequentialwaterbalance analysis in VID was an appropriate approach to accurately identify and quantify the most important waterbalance components while minimizing waterbalanceclosingerrors.