Optimal irrigation efficiency for maximum plant productivity and minimum water loss

Castilla-La Mancha in Central Spain is a semi-arid area of extremely high i nterannual and seasonal rainfall variability. Average annual rainfall for t he catchment of the Upper Guadiana using data from 60 rain gauges for Octob er 1956-September 1991 varied from a minimum of 326 mm in October 1982-Octo ber 1983 to a maximum of 642 mm in October 1968-September 1969. The mean an nual rainfall for the period was 495 mm with a coefficient of variation for annual rainfall of 26.4%. In addition to this the spatial variability of r ainfall is particularly high. For example total annual rainfall varied from 200 to 1200 mm for the hydrological year October 1968-September 1969 over a distance of only 50 km. The mean annual rainfall for all 60 stations for the period 1956-1991 was 460 mm with a coefficient for spatial variation of 15%. Dryland farming which relies on these uncertain precipitation inputs is a high risk activity [Tarjuelo, J.M., de Juan, J.A., Valiente, M., Garci a, P., 1996. Agric. Water Manage. 31, 145-163] and over recent decades inte nsive irrigation has increased dramatically in order that precipitation inp uts can be supplemented as required thereby allowing more stable agricultur al productivity. The paper uses a coupled hydrology and vegetation growth m odel - PATTERN [Mulligan, M., 1996. Modelling hydrology and vegetation chan ge in a degraded semi-arid environment. PhD. Thesis, University of London] to explore the relationship between irrigation and productivity for differe nt soils typical of the Upper Guadiana catchment. Analysis of the model res ults shows that irrigation efficiency is highly sensitive to both soil text ure and irrigation volume. Optimally efficient irrigation in terms of water losses occurs at the lowest volumes of applications. Fine grained soils ar e better suited to irrigation than coarse grained soils as losses to rechar ge are minimised. Coarse grained soils have large recharge losses and irrig ation is also more sensitive to small changes in texture for coarser soils. Irrigation losses through recharge are also very sensitive to the interann ual and spatial variability of rainfall. (C) 1999 Elsevier Science B.V. All rights reserved.