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

► The sequential water balance assessment helped to identify the main unknown water balance terms. ► The use of long hydrologic time-series achieved a better understanding of the system and added confidence to the estimations. ► A new elevated canal achieved water savings of 6.0 hm 3. ► It is important to consider ET a instead of ET c in the water balance of water-stressed irrigated areas. ► The final water balances defined presented annual closing errors within ±10%. Long-term analysis of hydrologic series in irrigated areas allows identifying the main water balance components, minimizing closing errors and assessing changes in the hydrologic regime. The main water inputs [irrigation ( I) and precipitation ( P)] and outputs [outflow ( Q) and potential (ET c) crop evapotranspiration] in the 4000-ha La Violada irrigation district (VID) (Ebro River Basin, Spain) were measured or estimated from 1995 to 2008. A first-step, simplified water balance assuming steady state conditions (with error ɛ = I + P − Q − ET c) showed that inputs were much lower than outputs in all years (average ɛ = −577 mm yr −1 or −33% closing error). A second-step, improved water balance with the inclusion of other inputs (municipal waste waters, canal releases and lateral surface runoff) and the estimation of crop's actual evapotranspiration (ET a) through a daily soil water balance reduced the average closing error to −13%. Since errors were always higher during the irrigated periods, when canals are full of water, a third-step, final water balance considered canal seepage (CS) as an additional input. The change in water storage in the system (Δ W) was also included in this step. CS and Δ W were estimated through a monthly soil–aquifer water balance, showing that CS was a significant component in VID. With the inclusion of CS and Δ W in the water balance equation, the 1998–2008 annual closing errors were within ±10% of total water outputs. This long-term, sequential water balance analysis in VID was an appropriate approach to accurately identify and quantify the most important water balance components while minimizing water balance closing errors.