Modulation of Compound Extremes of Low Soil Moisture and High Vapor Pressure Deficit by Irrigation in India

Abstract

Compound extremes of low soil moisture and high vapor pressure deficit (VPD) have implications on terrestrial carbon sequestration, vegetation growth, and net primary productivity (NPP). Yet, the role of irrigation on the occurrence of compound extremes remains unexplored in India. Here, we use satellite observations, reanalysis data sets, and high‐resolution simulations from the Weather Research and Forecasting (WRF) model to show that irrigation significantly reduces the frequency of compound extremes of low soil moisture and high VPD in India. The Indo‐Gangetic plain witnessed a significant (p < 0.05) increase in mean annual soil moisture while a substantial reduction in VPD during 1979–2018. Moreover, irrigation caused considerable cooling over the Indo‐Gangetic plain during 1982–2018. The increase in soil moisture and the decline in VPD partly contributed to a substantial rise in Normalized Difference Vegetation Index (NDVI) and NPP. We conducted high‐resolution simulations using the WRF model to examine the role of irrigation in modulating the frequency of compound extremes during 1979–2018. The WRF simulations under the irrigation‐on show a considerably reduced frequency of the compound extremes compared to the irrigation‐off (control) simulations. Irrigation modulates compound extremes by changing the land surface energy budget and planetary boundary layer height. Therefore, there is a need to consider irrigation for reliable projections of compound extremes under a warming climate in intensively irrigated regions.

Improved Water Savings and Reduction in Moist Heat Stress Caused by Efficient Irrigation

“…The substantial rise in irrigation water use has caused several environmental implications, including alteration of dry and moist heat (Ambika & Mishra, 2020; Mishra et al., 2020) and rapid groundwater depletion (Mishra et al., 2018). In addition, an extensive irrigation water withdrawal during the summer (April–May) significantly alters the surface energy and radiation budget (Ambika & Mishra, 2020, 2021; Douglas et al., 2006; Qian et al., 2013; Yang et al., 2019). Further, a significant increase in evaporative stress is observed along extensively irrigated Indo‐Gangetic plain (Figure S1b in Supporting Information S1).…”

“…Change in evaporative stress indicates phenological constraints acting on evaporation over the Indo‐Gangetic plain (Figure S1c in Supporting Information S1), which are associated with the changes in vapou due to the soil moisture desiccation (Zhou et al., 2019). For instance, lower sun‐induced chlorophyll fluorescence (SIF; Ambika & Mishra, 2021) indicates a decline in leaf and canopy photosynthetic rates when atmospheric VPD increases due to stomatal closure (Yuan et al., 2019). Overall, a massive irrigation expansion combined with the increased atmospheric water demands caused a rise in the irrigation water use in the Indo‐Gangetic Plain.…”

“…On the other hand, a significant (at 10% significance level) decrease in wet‐bulb temperature was found over the Indo‐Gangetic plain when switching from channel to drip irrigation system (Figures 5c and 5d). The extreme wet‐bulb temperature has increased due to irrigation (Krakauer et al., 2020; Mishra et al., 2020), while dry bulb temperature declined during the summer (Ambika & Mishra, 2021) in the Indo‐Gangetic Plain. However, switching from channel to drip irrigation in the Indo‐Gangetic Plain can decline wet‐bulb temperature and enhanced water savings over the Indo‐Gangetic Plain (Figure 5).…”

“…Irrigation expansion in India ensured food security for about 1.4 billion people, improved the socio‐economic condition, and increased Gross Domestic Product (GDP; Chen et al., 2019; Narayanamoorthy, 2011)s. Irrigation has been a significant contributor to improving the Indo‐Gangetic region's socio‐economic condition (Krishna Kumar et al., 2004; Narayanamoorthy & Deshpande, 2003). However, irrigation modulated regional climate, energy, and water budgets (Ambika & Mishra, 2019, 2020, 2021; Shah et al., 2019). Excessive moisture from agricultural expansion and irrigation combined with increased warming has affected human health, reduced labor performance, and led to economic loss (Kjellstrom et al., 2016; Venugopal et al., 2016).…”

Climate warming and summer monsoon breaks drive compound dry and hot extremes in India

“…We only considered the pentads for the summer monsoon season (June-September, JJAS) in our analysis. While previous studies used copula ( Zhou and Park Williams, 2019 ; Zhou and Zhang, 2019 ) to identify the compound extremes, we adopted a threshold-based approach that has been extensively used to examine compound extremes in India ( Sharma and Mujumdar, 2017 ; Mishra et al., 2020 ; Ambika and Mishra, 2021 ; Dash and Maity, 2021 ; Guntu and Agarwal, 2021 ). Since the pentad-based estimates smooth out short-term fluctuations in soil moisture and temperature, compound extremes were identified using the fixed thresholds for pentads.…”