Crop-specific seasonal estimates of irrigation-water demand in South Asia

Hester Biemans,Christian Siderius,Ashok Mishra,Bashir Ahmad

doi:10.5194/hess-20-1971-2016

Abstract

Abstract. Especially in the Himalayan headwaters of the main rivers in South Asia, shifts in runoff are expected as a result of a rapidly changing climate. In recent years, our insight into these shifts and their impact on water availability has increased. However, a similar detailed understanding of the seasonal pattern in water demand is surprisingly absent. This hampers a proper assessment of water stress and ways to cope and adapt. In this study, the seasonal pattern of irrigation-water demand resulting from the typical practice of multiple cropping in South Asia was accounted for by introducing double cropping with monsoon-dependent planting dates in a hydrology and vegetation model. Crop yields were calibrated to the latest state-level statistics of India, Pakistan, Bangladesh and Nepal. The improvements in seasonal land use and cropping periods lead to lower estimates of irrigation-water demand compared to previous model-based studies, despite the net irrigated area being higher. Crop irrigation-water demand differs sharply between seasons and regions; in Pakistan, winter (rabi) and monsoon summer (kharif) irrigation demands are almost equal, whereas in Bangladesh the rabi demand is ∼ 100 times higher. Moreover, the relative importance of irrigation supply versus rain decreases sharply from west to east. Given the size and importance of South Asia improved regional estimates of food production and its irrigation-water demand will also affect global estimates. In models used for global water resources and food-security assessments, processes like multiple cropping and monsoon-dependent planting dates should not be ignored.

Highlights

As global demand for food increases, water resources – one of the main resources for producing food – are becoming increasingly stressed
We aim to provide such spatially explicit, crop-specific seasonal estimates of water demand and crop production, using a revised version of the LundPotsdam-Jena managed Land (LPJmL) hydrology and vegetation model (Gerten et al, 2004), adjusted for the region
While Pakistan’s net irrigation demand is almost divided over the kharif and rabi seasons, India’s demand is skewed towards the rabi season; almost three-quarters of net irrigation demand in India occurs in this dry season

Summary

Introduction

As global demand for food increases, water resources – one of the main resources for producing food – are becoming increasingly stressed. Excess food production in recent years has obscured this bleak future; increases in both agricultural productivity and cropland extension have made the region food self-sufficient in its staple crops in recent decades. The resources that supported this increase – surface- and groundwater extracted for irrigation, land converted into cropland, increased use of nutrients and pesticides – are not unlimited. Groundwater levels are already falling rapidly in large parts of South Asia due to overexploitation (Rodell et al, 2009; Tiwari et al, 2009) and surface-water irrigation is reaching its limits (Biemans, 2012), costly river interlinking schemes aside (Bagla, 2014; Gupta and Deshpande, 2004). Higher temperatures and an expected higher variability in climate due to global warming further jeopardizes future food production in this region (Krishna Kumar et al, 2004; Mall et al, 2006; Moors et al, 2011)

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