Journal Title Title of Journal: Clim Dyn Search In Journal Title: Abbravation: Climate Dynamics Search In Journal Abbravation: Publisher Springer-Verlag Search In Publisher: DOI 10.1007/978-1-4939-0779-3_130-1 Search In DOI: ISSN 1432-0894 Search In ISSN:
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# Global effect of irrigation and its impact on the onset of the Indian summer monsoon

## Abstract

In a context of increased demand for food and of climate change, the water consumptions associated with the agricultural practice of irrigation focuses attention. In order to analyze the global influence of irrigation on the water cycle, the land surface model ORCHIDEE is coupled to the GCM LMDZ to simulate the impact of irrigation on climate. A 30-year simulation which takes into account irrigation is compared with a simulation which does not. Differences are usually not significant on average over all land surfaces but hydrological variables are significantly affected by irrigation over some of the main irrigated river basins. Significant impacts over the Mississippi river basin are shown to be contrasted between eastern and western regions. An increase in summer precipitation is simulated over the arid western region in association with enhanced evapotranspiration whereas a decrease in precipitation occurs over the wet eastern part of the basin. Over the Indian peninsula where irrigation is high during winter and spring, a delay of 6 days is found for the mean monsoon onset date when irrigation is activated, leading to a significant decrease in precipitation during May to July. Moreover, the higher decrease occurs in June when the water requirements by crops are maximum, exacerbating water scarcity in this region. A significant cooling of the land surfaces occurs during the period of high irrigation leading to a decrease of the land-sea heat contrast in June, which delays the monsoon onset.Simulations were performed using computational facilities of the Institut du Développement et des Ressources en Informatique Scientifique (IDRIS, CNRS, France). The authors would like to thank the European Union project WATCH (WATer and global CHange, contract No. 036946) for the financial support. Great thanks to Camille Risi (LMD, CNRS, France) for her useful comments on the English writing.The canopy resistance (including both bulk stomatal and leaf aerodynamic resistances), $$r_{sto_{v}}$$ (s/m) (Eq. 4), is derived by Lohammar et al. (1980) from Jarvis (1976). It depends on the net solar radiation (R net SW  (W/m2)) and the water vapor deficit of the air (δ c  (kg/m3)) which are limiting factors for transpiration, and it is inversely proportional to the LAI.Maximal water loss by crops, computed to estimate irrigation requirement, is derived from the effective transpiration parametrization under stress-free conditions, called potential transpiration, T v pot  (kg/m2/s). In these conditions, soil water is not limiting ($$U_{s_{v}}=1$$ in Eq. 3), the stomatal opening is no longer dependant on net solar radiation and water deficit of the air is zero (respectively $$\frac{R_{net}^{SW}+R_{0}}{R_{net}^{SW}}=1$$ and δ c  = 0 in Eq. 4). The stomatal resistance of the foliage is thus minimal (respectively $$r_{sto_{v}=}r_{sto_{v}}^{min}$$).

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