Spatial Pattern of Plant Transpiration Over China Constrained by Observations

Plant transpiration is a key flux of surface water loss to the atmosphere, determining the available surface water for ecosystem and human use. However, over China, the magnitude of transpiration and its spatial pattern remain poorly understood due to a lack of constraints from in situ observations. Here we compile 34 plot‐scale annual transpiration measurements across China to evaluate the performance of four transpiration products, and then produce a new constrained transpiration map for China. The transpiration map reveals an annual value of 209.0 mm yr−1, while the four transpiration products have a large spread of values (173.4–307.9 mm yr−1), especially in the wet southeastern regions. Land surface models exhibit large biases in modeled transpiration (−62.8% to 49.1%) compared to our constrained transpiration, and tend to underestimate transpiration in wet regions while overestimating it in dry regions. This behavior introduces bias in runoff projections which has ramifications for regional water resource management policies. Plain Language Summary Transpiration from plants is a very important part of the hydrological cycle but, in China, the way in which it varies from one place to another is poorly understood because there is a lack of direct measurements, especially in remote areas. We took 34 measurements made by other teams in different areas of China and combined these with four transpiration products to produce a more accurate map of transpiration across China. We found that the irrigation of crops, which is common in some areas, had a large effect on how transpiration varies from place to place. We compared our new map of transpiration with simulations made by the most up‐to‐date land surface models, in the situation where the concentration of CO2 in the atmosphere is increasing. They do not agree well and this disagreement leads to variations in runoff projects which will have implications for national water resource management. Key Points We produce a constrained transpiration map for China by combining 34 plot‐scale observations and four independent transpiration products Human irrigation plays a critical role in the spatial variation of transpiration Land surface models introduce bias in runoff projections due to poor performance of transpiration simulation