Although water availability strongly controls gross primary production (GPP), the impact of soil moisture content (wilting point) is poorly quantified on regional and global scales. In this study, we used 10-year observations of solar-induced chlorophyll fluorescence (SIF) from the GOSAT satellite to estimate the wilting point of a semiarid grassland on the Mongolian Plateau. Radiative-transfer model inversion and soil-vegetation-atmosphere transfer simulation were jointly conducted to distinguish the drought impacts on physiology from changes in leaf-canopy optical properties. We modified an existing inversion algorithm and the widely used SCOPE model to adequately evaluate dryland features, e.g., sparse canopy and strong convection. The modified model with retrieved parameters and calibrated to GOSAT SIF predicts realistic GPP values. We found that (1) the SIF yield estimated from GOSAT shows a clear sigmoidal pattern in relation to drought, and the estimated wilting point matches ground-based observations within ~0.01 m3 m-3 for the soil moisture content, (2) tuning the maximum carboxylation rate improves SIF prediction after considering changes in leaf-canopy optical properties, implying that GOSAT detected drought stress in leaf-level photosynthesis, and (3) the surface energy balance has significant impacts on the grassland’s SIF; the modified model reproduces observed SIF radiance well (mean bias = 0.004 mW m-2 nm-1 sr-1 in summer), whereas the original model predicts substantially low values under weak horizontal wind (unstable) conditions. Some model-observation mismatches in the SIF suggest that more research is needed for fluorescence parametrization (e.g., photoinhibition) and additional observation constraints.

In Japan, there has recently been an increasing call for forest thinning to conserve water resources from forested mountain catchments in terms of runoff during prolonged drought periods of the year. How their water balance and the resultant runoff are altered by forest thinning is examined using a combination of 8-year hydrological observations, 100-year meteorological data generator output, and a semi-process-based rainfall-runoff model. The rainfall-runoff model is developed based on TOPMODEL assuming that forest thinning has an impact on runoff primarily through an alteration in canopy interception. The main novelty in this analysis is that the availability of the generated 100-year meteorological data allows the investigations of the forest thinning impacts on mountain catchment water resources under the most severer drought conditions. The model is validated against runoff observations conducted at a forested mountain catchment in the Kanto region of Japan for the period 2010--2017. It is demonstrated that the model reproduces temporal variations in runoff and evapotranspiration at inter- and intra-annual time scales, resulting in well reproducing the observed flow duration curves. On the basis of projected flow duration curves for the 100-year, despite the large increase in an annual total runoff with ordinary intensifying thinning, low flow rates, i.e., water resources from the catchment in the drought period in the year, in both normal and drought years were impacted by the forest thinning to a lesser extent. Higher catchment water retention capacity appreciably enhanced the forest thinning effect on increasing available water resources.