Uncertainties in modelling the climate impact of irrigation

Abstract

Irrigation-based agriculture constitutes an essential factor for food security as well as fresh water resources and has a distinct impact on regional and global climate. Many issues related to irrigation's climate impact are addressed in studies that apply a wide range of models. These involve substantial uncertainties related to differences in the model's structure and its parametrizations on the one hand and the need for simplifying assumptions for the representation of irrigation on the other hand. To address these uncertainties, we used the Max Planck Institute for Meteorology's Earth System model into which a simple irrigation scheme was implemented. In order to estimate possible uncertainties with regard to the model's more general structure, we compared the climate impact of irrigation between three simulations that use different schemes for the land-surface-atmosphere coupling. Here, it can be shown that the choice of coupling scheme does not only affect the magnitude of possible impacts but even their direction. For example, when using a scheme that does not explicitly resolve spatial subgrid scale heterogeneity at the surface, irrigation reduces the atmospheric water content, even in heavily irrigated regions. Contrarily, in simulations that use a coupling scheme that resolves heterogeneity at the surface or even within the lowest layers of the atmosphere, irrigation increases the average atmospheric specific humidity. A second experiment targeted possible uncertainties related to the representation of irrigation characteristics. Here, in four simulations the irrigation effectiveness (controlled by the target soil moisture and the non-vegetated fraction of the grid box that receives irrigation) and the timing of delivery were varied. The second experiment shows that uncertainties related to the modelled irrigation characteristics, especially the irrigation effectiveness, are also substantial. In general the impact of irrigation on the state of the land surface is more than three times larger when assuming a low irrigation effectiveness than when a high effectiveness is assumed. For certain variables, such as the vertically integrated water vapour, the impact is almost an order of magnitude larger. The timing of irrigation also has non-negligible effects on the simulated climate impacts and it can strongly alter their seasonality.

The Role of Irrigation Expansion on Historical Climate Change: Insights From CMIP6

“…This suggests that the impact of irrigation on the simulated climate can differ between models despite identical irrigation implementation. These differences may originate from substantial differences in the model setup, structure and atmospheric parametrizations ( de Vrese & Hagemann, 2018). Although CESM2, GISS-E2-1-G, and NorESM2-LM implement an irrigation scheme, GISS.E2.1.G shows the strongest increase in LHF, SM, and LW net over all irrigation classes, followed by NorESM2-LM and CESM2.…”

Evaluation of three new surface irrigation parameterizations in the WRF-ARW v3.8.1 model: the Po Valley (Italy) case study

“…This discrepancy in the causes was ascribed to the fact that, in the previous version of the atmospheric model (CAM3), convection was very sensitive to the surface latent heat changes (Thiery et al, 2017). The effects of irrigation go beyond the surface cooling, as it affects the surface energy partition (e.g., Cook et al, 2015) and thus the atmospheric dynamics (Guimberteau et al, 2012;Saeed et al, 2009;Tuinenburg and de Vries, 2017;Douglas et al, 2009;Saeed et al, 2009;Lee et al, 2011), water vapor content (e.g., Boucher et al, 2004), and finally precipitation (Pielke and Zeng, 1989;Deangelis et al, 2010;Bonfils and Lobell, 2007;Puma and Cook, 2010). Some of the regional studies did not find a significant change in the cloud cover (Kueppers et al, 2007(Kueppers et al, , 2008Sorooshian et al, 2011;Qian et al, 2013), while others did (Aegerter et al, 2017;Krakauer et al, 2016).…”

Global assessment of the impact of irrigation on land surface temperature

“…The impact of irrigation on climate, especially temperature, has been assessed by various Earth system models, which have demonstrated that, despite a slight decrease in surface albedo, the net biophysical effect of irrigation is to cool surface temperature through the increase in evapotranspiration (ET) [1,2]. Modeling results are effective in presenting mechanistic understandings of the effects of irrigation on climate, but show high uncertainties in the sign, magnitude, and spatial distribution of the predicted effects, due to their heavy dependence on the model's structure and parameterization [3,4]. Observations from in-situ measurements (e.g., weather stations and field experiments) can provide local reliable evidence to verify the model results [5].…”