CLIMATE CHANGE AND AGRICULTURE Reliability of current Spanish irrigation designs in a changed climate: a case study A.UTSET 1 * AND B. DEL RÍO 2 1 Clean Earth Consultancy, Research and Development Department, Spain 2 Instituto Tecnológico Agrario de Castilla y Leon (ITACyL), Spain (Revised MS received 31 August 2010; Accepted 28 September 2010; First published online 10 December 2010) SUMMARY A very serious effort to modernize irrigation systems is being made in Spain, to reduce water and energy losses in an environmentally sustainable way. This is expensive and it is important that the new irrigation systems work properly over a long period. The systems have been designed taking into account historical evapotranspiration (ET) averages during the months of maximum demand, as well as the crop-specific ET values (Kc coefficients) of typical crops. However, the increase in ET rates due to global warming could mean that the capacity of these new and expensive irrigation systems to fulfil the crop water requirements may be exceeded in the near future. However, the expected increase in CO 2 concentration could diminish crop transpiration rates for similar water demands from the atmosphere, thereby reducing irrigation requirements. A methodology was developed in order to estimate crop water requirements under climate change conditions. The reliability of a new irrigation system designed in Valladolid, Northern Spain was tested. The regionalized climate change scenarios for Valladolid, provided by the National Institute of Meteorology, were used for the periods 2011–40, 2041–70 and 2071–2100 and the A2 and B2 emission scenarios were considered using the ECHAM and coupled general circulation model (CGCM) global circulation models. A historical series of daily meteorological data for Valladolid was used to generate statistical ET distributions through the LARS-WG generator. Simulations considered each of the above periods, global circulation models (GCM) and emission scenarios. Furthermore, the Kc of the typical irrigated crops of the zone (maize, potato and sugar beet) were reduced for each period, GCM and emission scenario according to the relationships between CO 2 concentrations and transpiration obtained by Kruijt et al.(2008). The results indicated that, on average, historical ET rates provide a sufficiently robust indicator to enable estimations of the crop ET in the future, particularly considering the CO 2 effect in reducing crop transpiration. However, ET variability is significantly increased after 2040, especially for the A2 emission scenario. The results show that ET variability rather than global increase is the most serious risk that current irrigation systems must face in the near future in Northern Spain, as consequence of climate change. Such variability should be included in irrigation designs. INTRODUCTION As stated by the last report on climate change effects (IPCC 2007), Mediterranean regions can expect higher temperatures, less water availability and an increase in drought frequency. Irrigated agriculture could face serious risks in southern Europe in the near future, due to water scarcity and the potential increase in evapotranspiration (ET) rates (EEA 2007). This is an important problem in Spain, where irrigation uses about 0·70 of the available water (MAPA 2005). According to Iglesias et al.(2005), some Spanish regions can expect up to 50% reduction in water * To whom all correspondence should be addressed. Email: autset@clnearth.com Journal of Agricultural Science (2011), 149, 171–183. © Cambridge University Press 2010 doi:10.1017/S0021859610001073 171