SEBAL for detecting spatial variation of water productivity and scope for improvement in eight irrigated wheat systems Sander J. Zwart *, Wim G.M. Bastiaanssen WaterWatch, Generaal Foulkesweg 28, 6703 BS Wageningen, The Netherlands 1. Introduction Due to the rapid growth in world population, the pressure on water resources is increasing (Rijsberman, 2006). In the future, less water will be available for agricultural production due to competition with the industrial and domestic sectors, while at the same time food production must be increased to feed the growing population. In systems where water is becoming the limiting factor, agricultural production should be expressed per unit of water consumed instead of production expressed per unit land. It is inevitable that the production per unit water consumed, the water productivity, must be increased to meet this challenge (see e.g. Kijne et al., 2003; Molden et al., 2007). Spatial information on water use, crop production and water productivity will play a vital role for water managers to assess where scarce water resources are wasted and where in a given region the water productivity can be improved. Currently, information on water productivity is often only available from experiments on a single field, so that results are limited to the local (environmental) conditions that can vary from year-to-year and to the specific soil, crop and water management practices. Crop water consumption cannot be routinely measured, and this hampers the introduction of the concept of crop water productivity per unit of water depletion agricultural water management 89 (2007) 287–296 article info Article history: Accepted 3 February 2007 Keywords: SEBAL Water productivity Wheat Evapotranspiration Crop production Water saving abstract A methodology has been developed to quantify spatial variation of crop yield, evapotran- spiration (ET) and water productivity (WP ET ) using the SEBAL algorithm and high and low resolution satellite images. SEBAL-based ET estimates were validated over an irrigated, wheat dominated area in the Yaqui Valley, Mexico and proved to be accurate (8.8% difference for 110 days). Estimated average wheat yields in Yaqui Valley of 5.5 t ha 1 were well within the range of measured yields reported in the literature. Measured wheat yields in 24 farmers’ fields in Sirsa district, India, were 0.4 t ha 1 higher than SEBAL estimated wheat yields. Area average WP ET in the Yaqui Valley was 1.37 kg m 3 and could be considered to be high as compared to other irrigated systems around the world where the same methodology was applied. A higher average WP ET was found in Egypt’s Nile Delta (1.52 kg m 3 ), Kings County (CA), USA (1.44 kg m 3 ) and in Oldambt, The Netherlands (1.39 kg m 3 ). The spatial variability of WP ET within low productivity systems (CV = 0.33) is higher than in high productivity systems (CV = 0.05) because water supply in the former case is uncertain and farming conditions are sub-optimal. The high CV found in areas with low WP ET indicates that there is considerable scope for improvement. The average scope for improve- ment in eight systems was 14%, indicating that 14% ET reduction can be achieved while maintaining the same yield. It is concluded that the proposed methodology is accurate and that better knowledge of the spatial variation of WP ET provides valuable information for achieving local water conservation practices in irrigated wheat. # 2007 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +31 3174 23 602; fax: +31 344 693827. E-mail address: s.zwart@waterwatch.nl (S.J. Zwart). available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/agwat 0378-3774/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2007.02.002