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Evapotranspiration (ET) represents the largest water loss flux in drylands,but ET and its partition into plant transpiration (T) and soil evaporation (E)are poorly quantified, especially at fine temporal scales. Physically-basedremote sensing models relying on sensible heat flux estimates, like thetwo-source energy balance model, could benefit from considering more explicitlythe key effect of stomatal regulation on dryland ET. The objective of thisstudy is to assess the value of solar-induced chlorophyll fluorescence (SIF), aproxy for photosynthesis, to constrain the canopy conductance (Gc) of anoptimal stomatal model within a two-source energy balance model in drylands. Weassessed our ET estimation using in situ eddy covariance GPP as a benchmark,and compared with results from using the Contiguous solar-induced chlorophyllfluorescence (CSIF) remote sensing product instead of GPP, with and without theeffect of root-zone soil moisture on the Gc. The estimated ET was robust acrossfour steppes and two tree-grass dryland ecosystem. Comparison of ET simulatedagainst in situ GPP yielded an average R2 of 0.73 (0.86) and RMSE of 0.031(0.36) mm at half-hourly (daily) timescale. Including explicitly the soilmoisture effect on Gc, increased the R2 to 0.76 (0.89). For the CSIF model, theaverage R2 for ET estimates also improved when including the effect of soilmoisture: from 0.65 (0.79) to 0.71 (0.84), with RMSE ranging between 0.023(0.22) and 0.043 (0.54) mm depending on the site. Our results demonstrate thecapacity of SIF to estimate subdaily and daily ET fluxes under very low ETconditions. SIF can provide effective vegetation signals to constrain stomatalconductance and partition ET into T and E in drylands. This approach could beextended for regional estimates using remote sensing SIF estimates such asCSIF, TROPOMI-SIF, or the upcoming FLEX mission, among others.

Dryland evapotranspiration from remote sensing solar-induced chlorophyll  fluorescence: constraining an optimal stomatal model within a two-source  energy balance model