Memory of irrigation effects on hydroclimate and its modeling challenge

Abstract

Irrigation modifies land-surface water and energy budgets, and also influences weather and climate. However, current earth-system models, used for weather prediction and climate projection, are still in their infancy stage to consider irrigation effects. This study used long-term data collected from two contrasting (irrigated and rainfed) nearby maize-soybean rotation fields, to study the effects of irrigation memory on local hydroclimate. For a 12 year average, irrigation decreases summer surface-air temperature by less than 1 • C and increases surface humidity by 0.52 g kg −1 . The irrigation cooling effect is more pronounced and longer lasting for maize than for soybean. Irrigation reduces maximum, minimum, and averaged temperature over maize by more than 0.5 • C for the first six days after irrigation, but its temperature effect over soybean is mixed and negligible two or three days after irrigation. Irrigation increases near-surface humidity over maize by about 1 g kg −1 up to ten days and increases surface humidity over soybean (∼ 0.8 g kg −1 ) with a similar memory. These differing effects of irrigation memory on temperature and humidity are associated with respective changes in the surface sensible and latent heat fluxes for maize and soybean. These findings highlight great need and challenges for earth-system models to realistically simulate how irrigation effects vary with crop species and with crop growth stages, and to capture complex interactions between agricultural management and water-system components (crop transpiration, precipitation, river, reservoirs, lakes, groundwater, etc.) at various spatial and temporal scales.

Lessons Learned From Modeling Irrigation From Field to Regional Scales

“…The land surface hydrometeorological conditions over Nebraska and LMRB were greatly modified by irrigation, including the cooling effect of 0.8–1.4 K, moistening effect of 1.2–2.4 g/kg, reduction in sensible heat fluxes by 60 to105 W/m 2 , and increase in latent heat fluxes by 75 to 120 W/m 2 . Those local effects were consistent with the observational study by Chen et al (). Note that HRLDAS‐simulated irrigation effects concentrated over the irrigated lands due to its offline setting, and future coupling runs with atmospheric models are necessary to improve the understanding of irrigation effects on regional land‐atmosphere interactions.…”

“…In August when the differences between the OPTM2D and NOIRR runs generally reached maxima, the modeled near‐surface air temperature decreased by 0.8–1.2 K in southeastern Nebraska and by up to ~1.4 K in eastern Arkansas, while 2‐m air humidity increased around 1.2–1.8 g/kg and even greater than 2.4 g/kg over the two above heavily irrigated areas. The degree of cooling or moistening shown here was highly correlated with the total irrigation water amounts (Figures a and b) and was largely on par with Chen et al ().…”

“…As demonstrated by many previous studies (e.g., Chen et al, 2018;Cook et al, 2015;Haddeland et al, 2006;Leng et al, 2013;Ozdogan et al, 2010;Pei et al, 2016;Qian et al, 2013;Sacks et al, 2009), irrigation has great influences on land surface water and energy budgets, including the cooling and moistening effects and the repartitioning of sensible and latent heat fluxes shown in Figure9 for the OPTM2D run. In August when the differences between the OPTM2D and NOIRR runs generally reached maxima, the modeled near-surface air temperature decreased by 0.8-1.2 K in southeastern Nebraska and by up tõ 1.4 K in eastern Arkansas, while 2-m air humidity increased around 1.2-1.8 g/kg and even greater than 2.4 g/kg over the two above heavily irrigated areas.…”

“…They are characterized by the same silty‐clay‐loam soils and undergo the same rotation of maize and soybean. Except for the lower planting density at the rainfed USNe3 site, the only meaningful difference between the two sites for a given year is irrigation (Chen et al, ; Verma et al, ). More detailed site information can be found in Verma et al () and Suyker and Verma ().…”

“…Observations (e.g., Bonfils & Lobell, 2007;Chen et al, 2018;Mahmood et al, 2006) and model simulations (e.g., Boucher et al, 2004;de Rosnay et al, 2003;Haddeland et al, 2006;Kueppers et al, 2007;Leng et al, 2013;Ozdogan et al, 2010;Pokhrel et al, 2012;Sacks et al, 2009) have demonstrated the important role of irrigation in modifying land surface water and energy budgets and subsequently local and regional weather and climate. For instance, irrigation reduced near-surface air temperature by decreasing (increasing) surface sensible (latent) heat fluxes (e.g., Chen et al, 2018;Cook et al, 2015;Haddeland et al, 2006;Leng et al, 2013;Ozdogan et al, 2010;Pei et al, 2016;Qian et al, 2013;Sacks et al, 2009). Wetter soils and higher latent heat fluxes led to higher atmospheric water vapor (Boucher et al, 2004), which in turn could enhance cloud cover and downwind precipitation (Cook et al, 2015;Pei et al, 2016;Qian et al, 2013).…”