How does irrigation alter the water, carbon, and nitrogen budgets in a large endorheic river basin?

Irrigation can substantially alter the hydrologic and biogeochemical cycles in river basins. However, little attention has been given to the quantitative evaluation of the effects of irrigation on both of these cycles. In this study, we utilized the latest version of the Community Land Model (CLM5) to assess the effects of irrigation on the water, carbon, and nitrogen budgets in a cropland area of a large endorheic river basin (Heihe River Basin, HRB) in northwestern China and further identified the contributions of each type of crop to the abovementioned budgets. The model has been validated against local flux towers, remote sensing products, and reanalysis datasets, demonstrating the capability of CLM5 in capturing the temporal and spatial characteristics of water, carbon, and nitrogen at the basin scale. Two numerical experiments, one with irrigation and one without it, were conducted to explore the relationships between water and carbon/nitrogen budgets for the period from 2000 to 2018. The simulation results showed that the greatest amount of irrigation was applied in the summer (>50 mm month⁻¹) and then declined to a low level during the other seasons. Evapotranspiration (ET) in the cropland area sharply increased from 106 mm year⁻¹ to 508 mm year⁻¹, with an increase in surface runoff and baseflow after irrigation. Both irrigated gross primary production (GPP), net ecosystem exchange (NEE) and water use efficiency (WUE) peaked in the summer, which corresponded to the crop growing season. A shift in NEE from 6 gC m⁻² year⁻¹ to –229 gC m⁻² year⁻¹ indicated that the cropland area in the HRB switched from being a carbon source to a carbon sink because of irrigation. The corn area presented the largest increase/decrease in GPP/NEE after irrigation, whereas the lowest increase/decrease in GPP/NEE was observed in the cotton area. Total ecosystem carbon (TEC) and total ecosystem nitrogen (TEN) markedly increased as a result of irrigation, with increases of 3.29 × 10¹³ gC and 2.96 × 10¹² gN, respectively, throughout the year in the cropland area. Overall, our study demonstrates that irrigation has substantial impacts on water, carbon, and nitrogen budgets in the cropland area of the HRB and provides important insights into the application of CLM5 in an arid basin with multiple crop types.