Satellite observations show that irrigation cools land surface temperature (LST) across China by about 1.15 K during the day on average, exceeding 6 K in arid-zone growing seasons, but this cooling is a net signal whose physical decomposition remains incomplete. Yang, Huang and Tang (2020), who established this observational record with their ICE detection (ICED) method, explicitly left open the process-level attribution question: how much of the observed irrigated-minus-non-irrigated LST contrast is produced by increased evapotranspiration, how much by cloud-radiative feedbacks, and how much by co-occurring management practices. No study provides a complete, observationally constrained partition of the all-China LST cooling into evaporative and cloud-radiative contributions with management confounds separated.

Background

Irrigation is the most extensive deliberate modification of the land surface water cycle: applying water to cropland raises soil moisture and plant growth, which changes how the surface exchanges energy with the atmosphere. Land surface temperature (LST) - the radiometric skin temperature of the ground, measurable from satellites such as MODIS (Moderate Resolution Imaging Spectroradiometer, the instrument on the Terra and Aqua satellites widely used for LST retrievals) - is the variable that ties this exchange to climate, because it responds directly to the surface energy balance: net absorbed radiation is split between latent heat flux (energy carried away by evapotranspiration, the sum of soil evaporation and plant transpiration), sensible heat flux (direct heating of the air), and ground heat storage. When a field is irrigated, a larger share of the available energy leaves as latent heat, and the surface cools; this is the irrigation cooling effect (ICE). Because this cooling can mask greenhouse-gas warming in heavily irrigated regions, quantifying it matters for interpreting observed temperature trends.

Yang, Huang and Tang (2020) provided the first comprehensive satellite-based assessment of the ICE on LST across China. They introduced an ICE detection method (ICED) that compares MODIS LST between irrigated areas and adjacent non-irrigated areas inside self-adaptive moving windows - analysis neighborhoods whose position and size adjust so that the irrigated pixels and their non-irrigated reference pixels share the same background climate. Irrigation cooled daytime LST by 1.15 K and nighttime LST by 0.13 K on average across irrigated China; the effect exceeded 6 K in the growing season in the arid climate zone and was negligible in the humid zone.

The authors were explicit about what their net LST contrast could not decide. First, the mechanisms behind the contrast were not partitioned: increased evapotranspiration is the presumed driver, but irrigation can also moisten and cool the atmospheric boundary layer (the lowest layer of the atmosphere, which exchanges heat, moisture, and momentum directly with the land surface), altering cloud cover and the incoming shortwave and longwave radiation reaching the surface (a cloud-radiative feedback), and the relative contributions of these pathways were not quantified; nor could irrigation be separated from other management practices that co-vary with it, such as fertilization and harvest timing. Second, the irrigated-area map was static, so nothing could be said about how irrigation expansion or changing irrigation practice alters LST trends. Third, residual land-cover heterogeneity (some regions required non-irrigated grassland as reference where non-irrigated cropland was scarce) left unquantified uncertainty. Fourth, how the surface cooling propagates to near-surface air temperature, humidity, precipitation, and heat stress in hot-humid conditions (humid heat stress, the combined physiological burden of heat and atmospheric moisture, which rises when moist air restricts the body's evaporative cooling) was not assessed.

Subsequent work has absorbed much of this agenda. Regional climate modeling over China with and without irrigation has quantified effects on 2-m air temperature (near-surface air temperature at the standard 2-meter measurement height), surface fluxes, and precipitation (Liu et al., 2021), and on temperature extremes, attributing them mainly to repartitioning of sensible and latent heat fluxes (Li et al., 2023). The greening that irrigation induces - and the transpiration that comes with it - was shown to contribute roughly one third of the springtime cooling over the North China Plain (a major, intensively irrigated farming region of eastern China) (Liu and Wang, 2023). Irrigation-expansion cooling has been characterized from satellites for Northwest China (Zhang et al., 2023) and Inner Mongolia (Li et al., 2024), the temporal weakening of cooling under water-saving techniques has been documented nationally with surface energy balance analysis (Zhang et al., 2025), and the drylands' biophysical responses have been quantified jointly for LST, evapotranspiration, greenness, and yield (Zi et al., 2025). The propagation question has been answered directly: combining station records and regional simulations, Yan et al. (2025) showed that irrigation in China cools daily maximum air temperature while moistening the air enough to intensify humid heat stress, and Yuan et al. (2026) found that historical irrigation expansion in the North China Plain mitigated spring heat stress while exacerbating nighttime moist heat stress. What still stands unresolved is the attribution core: an observationally constrained decomposition of the observed all-China LST contrast itself into its physical pathways.

Problem Statement

Provide a process-level, observationally constrained attribution of the irrigation cooling effect on land surface temperature across China, resolving the attribution item that Yang, Huang and Tang (2020) left open. Specifically, quantitatively partition the observed irrigated-minus-adjacent-non-irrigated LST contrast (as detected by the ICED moving-window method, the satellite comparison of irrigated and adjacent non-irrigated land introduced by Yang, Huang and Tang (2020), or an equivalent observational design) into: (i) the evaporative pathway, i.e., increased latent heat flux (optionally further decomposed into soil evaporation and plant transpiration); (ii) the cloud-radiative pathway, i.e., irrigation-induced changes in cloud cover and incoming shortwave and longwave radiation at the surface; and (iii) the contributions from co-occurring management practices (such as fertilization, cultivar choice, and harvest timing), identified independently of the evaporative and cloud-radiative pathways rather than treated as the arithmetic remainder of the partition. Residual land-cover heterogeneity (for example, the non-irrigated grassland reference pixels used where non-irrigated cropland is scarce) is a detection-level confounder to be bounded within the stated uncertainty analysis rather than a mandatory partition output. The mandatory output is the three-way partition (i)-(iii), delivered with quantified uncertainties for each of the three precipitation-defined climate zones used by Yang, Huang and Tang (2020) - arid (annual precipitation <400 mm), semi-arid/humid (400-800 mm), and humid (>800 mm) - and for the growing season (May-September) and the non-growing season (October-April); the soil-evaporation/transpiration decomposition of (i) is an optional refinement.

References

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