Reply to Comment on “Improved Water Savings and Reduction in Moist Heat Stress Caused by Efficient Irrigation” by Meetpal S. Kukal

Irrigation has a considerable influence on land surface energy and water budgets. Irrigation‐related processes influence energy partitioning, planetary boundary layer, land surface temperature, evapotranspiration, and runoff. However, observations related to irrigation and its influence on land surface hydrology and climate are limited. Among the limited observations available, most are from the field scale experiments that do not provide information on the regional scale impacts of irrigation. While land surface and climate models do not represent the complexity of field‐scale irrigation, these modeling experiments provide valuable insights into regional‐scale effects. We respond to a recent comment on our modeling study of irrigation impacts on regional climate. We acknowledge the limitations of our modeling experiments. However, we aimed to provide an experimental framework to understand the role of improved irrigation efficiency in water savings and in reducing moist heat stress at the regional scale.

• Lack of observations and field-scale irrigation complexity play a vital role in modeling the irrigation impacts on water and energy cycles • Land surface hydrology and climate models provide valuable insights into the irrigation influence at regional scales • Representation of irrigation processes in the models is crucial to examining water and energy cycle changes Correspondence to: A. K. Ambika and V. Mishra, a.anukesh@iitgn.ac.in; vmishra@iitgn.ac.in Citation: Ambika, A. K., & Mishra, V. (2023). Reply to comment on "improved water savings and reduction in moist heat stress caused by efficient irrigation" by Meetpal S. Kukal

REPLY
without being stressed or adding excess water. The drip irrigation scheme uses 37% of the irrigation water required for the flood scheme (Evans & Zaitchik, 2008). Moreover, evapotranspiration (ET) is about one-third less in drip than in flood irrigation. The decrease in ET in drip irrigation is mainly due to bare soil evaporation. We implemented the drip and channel irrigation methods based on the water loss. However, our study's application of irrigation schemes did not aim to resemble the technique used in the field experiments (Panigrahi et al., 2021;Parthasarathi et al., 2018;Rao et al., 2017). We acknowledge the limitation of the implemented schemes in isolating the evaporation from canopy and water ponding in some crops (e.g., rice), which can be considered for the parametrization in the regional models ( Figure 1).
MSK also commented on the application rate, irrigation frequency, and depth in channel and drip irrigation schemes used in our study. We have obtained the irrigation water requirement from the Global Reconstructed Water (GRW) data, and the schemes in our analysis considered the daily application of irrigation water from 6-9 a.m. during the irrigation period. However, considering the water demand in the Indo-Gangetic plain during the summer, the same amount of water is applied in drip and channel methods. We did not quantify the optimal water requirements for both irrigation techniques. We used the available water required from GRW data and converted that to daily water requirement. Therefore, efficiency is related to differences in evaporation and not from the water applied. Drip irrigation, in general, is designed to provide an optimal amount of water to maintain transpiration losses without stress and the addition of excess water. The most efficient system uses a soil moisture sensor to keep track of moisture deficit at crop roots and add water to the near-root region rather than the whole field (Evans & Zaitchik, 2008). However, that was not possible in the WRF simulation; instead, the current scheme ensures that there is always enough water for the vegetation to transpire without water limitations.
The other concern raised by MSK is on the representation of crop water use as decomposing evaporation and transpiration are critical for various irrigation choices. However, the comment is not directly relevant to our study. Using the WRF simulations, we examined drip irrigation's role in modulating moist heat in the heavily irrigated region (Indo-Gangetic Plain). Previous studies (Evans & Zaitchik, 2008;Grafton et al., 2018;Zhang et al., 2019) highlighted that drip irrigation consistently has a lesser evapotranspiration rate than channel/flooding irrigation. Overall, increased latent heat increases evapotranspiration and relative humidity in the irrigated regions. As our aim was not to quantify the role of transpiration and evaporation separately on moist heat stress, the decomposition of the two components was not considered.
We are entirely aware of the likelihood of transition from the channel to the drip irrigation system due to technicality, infrastructure, water availability, crop type, and soil properties. However, our main aim was to examine if improved irrigation efficiency can reduce moist heat and water-saving benefits. However, field studies have reported that irrigation efficiency, crop yield, and water conservation significantly increased from switching from conventional irrigation methods to drip irrigation systems. For example, the dry-seeded experiment conducted by Sharda et al. (2017) at Punjab Technical University resulted in a higher grain yield (7.34-8.01 t ha −1 ) than the flood irrigation (6.63-7.60 t ha −1 ) with overall water savings of more than 40%. Other studies (Panigrahi et al., 2021;Parthasarathi et al., 2018;Rao et al., 2017) also showed significant water savings through drip irrigation. Therefore, through our simulations, we highlighted that large-scale improvements in irrigation efficiency could reduce moist heat stress, notwithstanding the challenges associated with adapting drip irrigation. Our modeling experiments were based on certain assumptions and limitations. We did not aim to replicate the field scale field analysis. The irrigation parameterization in WRF is consistent with the earlier implementation of the irrigation scheme by Lawston et al. (2015). We aimed to understand the reduction/aggravation of moist heat with varying evaporative fractions in different irrigation techniques. The impacts of irrigation coupled with vegetation dynamics were out of the scope of our study due to the lack of representation of dynamic vegetation in the model.
We agree that summer crops are grown only in a small fraction of the Indo-Gangetic Plain compared to the two main crop-growing seasons. However, the area under the summer crops (especially hybrid maize) has substantially increased in the last two decades. In addition, summer temperature has also increased in several parts of the country. The economic and Statistical Organization (Economic and Statical Organisation, 2021) of Punjab reports that important crops like moong, groundnut, cotton, potato, and sugarcane have a February to June Sowing period. Further, wheat, gram, lentil, potato (winter), saffron, linseed, and sunflower have a harvesting period of April-May (Economic and Statical Organisation, 2021). Most of the Indo-Gangetic plain has been irrigated extensively during the summer, which is primarily driven by groundwater pumping, including April and May. Therefore, considering April-May would be an ideal condition to understand the impact of efficient irrigation on reducing heat stress and water conservation. Moreover, Wouters et al. (2022) reported that the morning dry soil attenuates afternoon heat stress by ∼5%, mainly due to reduced surface evaporation and entrainment of dry air aloft. The mean vegetation health during different seasons shows robust vegetation health during the MAM season ( Figure 2). Furthermore, soil moisture is also relatively higher during the MAM season over the Indo-Gangetic plain. The increase in NDVI and soil moisture in the Indo-Gangetic plain during the MAM season is significant. As discussed above, considerable cropping activity occurs during MAM for the Indo-Gangetic table. Therefore, we examined the impacts of irrigation efficiency on moist heat stress during summer. However, a similar analysis can be performed for the summer monsoon season. We feel that our study's conclusions (efficient irrigation reduces moist heat stress) would remain the same.
MSK stated that irrigation efficiency does not translate to water saving based on various global and regional studies. Moreover, asserting that basin-scale water accounting and farmers' behavior are crucial determinants of higher irrigation efficiency. We used conventional (channel) and efficient (drip) irrigation to examine the role of efficient irrigation in reducing moist heat and water savings. We observed that switching from conventional (channel) to efficient (drip) irrigation leads to moderate warming (∼0.2°C) and a significant decrease in specific humidity over the Indo-Gangetic Plain. The reduction in specific humidity due to efficient irrigation considerably lowers moist heat stress (wet-bulb temperature) and increases water savings. Our findings show the double benefits of efficient irrigation to curb the rapidly declining groundwater and increased moist heat stress in the region. Our study did not aim to consider the behavioral aspects related to irrigation efficiency translation to water savings.