Journal Article

Attributing Urban Evapotranspiration From Eddy‐Covariance to Surface Cover: Bottom‐Up Versus Top‐Down

Vulova, S., Jongen, H. J., Haacke, N., Teuling, A. J., Meier, F., Steeneveld, G. J., Jansen, F. A., Tetzlaff, D., Kleinschmit, B.

2024

Overview

Evapotranspiration (ET)$(ET)$is a key process in the hydrological cycle that can help mitigate urban heat. ET$ET$depends on the surface cover, as the surface affects the partitioning of precipitation between runoff and evapotranspiration. In urban neighborhoods, this surface cover is highly heterogeneous. The resulting neighborhood‐scale ET$ET$can be observed with eddy‐covariance systems. However, these observations represent the signal from wind‐ and stability‐dependent footprints resulting in a continuously changing contribution of surface cover types to the observation. This continuous change prevents quantifying the contribution of the surface cover types to neighborhood ET$ET$and their hourly dynamics. Here, we disentangle this neighborhood‐scale ET$ET$at two sites in Berlin attributing the patch‐scale ET$ET$dynamics to the four major surface cover types in the footprint: impervious surfaces, low vegetation, high vegetation, and open water. From the bottom‐up, we reconstruct neighborhood ET$ET$based on patch‐scale observations and conceptual models. Alternatively, we start top‐down and attribute neighborhood ET$ET$to the surface cover types solving a system of equations for three eddy‐covariance systems. Although data requirements for the bottom‐up approach are met more frequently, both approaches indicate that vegetation is responsible for more ET$ET$than proportional to its surface fraction in the footprint related to the large evaporating surface compared to the ground surface. Evaporation from impervious surfaces cannot be neglected, although it is less than from vegetation due to limited water availability. The limited water availability causes impervious surfaces to cease evaporation hours after rainfall, while vegetation and open water sustain ET$ET$for extended periods. Plain Language Summary Different types of surfaces, like grass, trees, pavement, and open water, affect how rainwater is divided between evaporation and runoff. In cities with lots of pavement and buildings, more water runs off than in natural areas leaving less water for evaporation. Measurement towers have been observing the evaporation from whole neighborhoods, but separating the effects of different surfaces is hard. In our study, we figure out how much each surface type contributes to evaporation with two methods: one starting from the separate surfaces and rebuilding the neighborhood evaporation, and the other starting with the neighborhood evaporation and breaking it down into evaporation from each surface. Both ways showed that plants evaporate more than proportionally to their surface area, but even built surfaces like pavement evaporate. Our findings confirm that more plants lead to more evaporation, but built surfaces cannot be ignored. This information can help urban planners create cities that manage water better, making cities nicer places to live. Key Points Neighborhood ET is reconstructed from patch‐scale data (bottom‐up) and disentangled attributing ET to four surface types (top‐down) The neighborhood and patch scale are connected through half‐hourly‐varying eddy‐covariance footprints ET dynamics after rainfall reveal that water limitation drives differences between surface cover types

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Cities / Covariance / Data requirements / eddy covariance / eddy‐covariance observations / Evaporation / Evapotranspiration / footprint modeling / heat / Hydrologic cycle / Hydrologic processes / Hydrological cycle / Information management / Neighborhoods / Pavements / Precipitation / Rain / Rain water / Rainfall / Runoff / surface cover / Surface stability / urban climate / urban hydrology / Urban planning / Urban runoff / Vegetation / Water availability / Water consumption