Mapping the total volumetric irrigation water requirements in England and Wales

The net volumetric (m(3)) irrigation water requirements for the main crop categories currently irrigated in England and Wales have been calculated and mapped within a geographic information system (GIS). The procedure developed by Knox et al. (1996, Agric. Water Manage., 31: 1-15) for maincrop potatoes (Solanum tuberosum) was extended to cater for the other crops currently irrigated. The annual irrigation needs (mm) for the eight major irrigated crop categories, grown on three contrasting soil types at 11 representative weather stations, were determined using a daily water balance irrigation scheduling model. The results were correlated with existing national datasets of climate, current land use, soils and irrigation practice, to generate volumetric (m(3)) irrigation water requirement maps at 2 km resolution. The total net volumetric irrigation water requirements for a UK 'design' dry year (defined as the requirement with a 20% probability of exceedance) are estimated to be 140 x 10(6) m(3) for the eight main crop categories currently irrigated and the 1994 cropping pattern. Previous theoretical dry year demand estimates, using scheduling models and large agroclimatic areas, were 109 x 10(6) m(3) and 222 x 10(6) m(3). The irrigation demand for other crops grown in the open would typically add another 4%. The procedure has been validated nationally, by comparing the calculated dry year demand for 1990 against government irrigation survey returns for 1990, for each crop category, and regionally against National Rivers Authority (NRA) abstraction records for 1990, for each NRA Region. The estimates obtained agree well with the reported distribution between crops and between regions. The most recent actual 'dry' year for which comparative data are available is 1990. It is estimated that the dry year requirements for the 1990 land use would have been 148 x 10(6) m(3). Although farmer demand, actual abstractions and crop requirements are not necessarily the same, irrigation survey returns to the Government indicated that 134 x 10(6) m(3) were actually applied, and the NRA estimated from meter returns that 138 x 10(6) m(3) were abstracted. It is noted, however, that some abstraction restrictions were in force, the scope of the data is slightly different and all figures contain inaccuracies. Potential applications for improving irrigation demand management and water conservation at regional and catchment levels are discussed with reference to two contrasting regions.