MAPPING THE TOTAL VOLUMETRIC IRRIGATION WATER REQUIREMENTS IN ENGLANDAND WALES

The net volumetric (m(3)) irrigation water requirements for the main c rop categories currently irrigated in England and Wales have been calc ulated and mapped within a geographic information system (GIS). The pr ocedure 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 contrast ing soil types at 11 representative weather stations, were determined using a daily water balance irrigation scheduling model. The results w ere correlated with existing national datasets of climate, current lan d use, soils and irrigation practice, to generate volumetric (m(3)) ir rigation water requirement maps at 2 km resolution. The total net volu metric irrigation water requirements for a UK 'design' dry year (defin ed as the requirement with a 20% probability of exceedance) are estima ted to be 140 x 10(6) m(3) for the eight main crop categories currentl y irrigated and the 1994 cropping pattern. Previous theoretical dry ye ar demand estimates, using scheduling models and large agroclimatic ar eas, 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 Rive rs Authority (NRA) abstraction records for 1990, for each NRA Region. The estimates obtained agree well with the reported distribution betwe en crops and between regions. The most recent actual 'dry' year for wh ich comparative data are available is 1990. It is estimated that the d ry year requirements for the 1990 land use would have been 148 x 10(6) m(3). Although farmer demand, actual abstractions and crop requiremen ts are not necessarily the same, irrigation survey returns to the Gove rnment 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 for ce, the scope of the data is slightly different and all figures contai n inaccuracies. Potential applications for improving irrigation demand management and water conservation at regional and catchment levels ar e discussed with reference to two contrasting regions.