Gonzalo Yáñez‐Morroni

A companion paper in this Special Issue reviewed the development of a hydrological model of the Silala River basin, using long‐term data (1969–1992) to determine the basin's water balance and plausible groundwater recharge scenarios. In the context of a remote river basin with limited in situ data, this article reviews the potential of in situ and remotely collected data, available for a relatively short period (2018–2019), to validate various aspects of the hydrological model performance. These include the spatiotemporal evolution of snow cover areal fraction (SCF), actual evapotranspiration (ET_a) in the basin's extensive alluvial deposits, and wetland ET_a. The observed SCF dynamics are well represented by the model at annual and monthly timescales, with monthly mean simulated SCF biases between −5.6% and 8%. At daily timescales, the model successfully captures snowstorm occurrence, although there are limitations on snow spatial patterning. Simulated ET_a over alluvial deposits agrees with in situ observations during periods of high ET_a, although the simulated values underestimate site‐specific observations during low ET_a periods, due to the presence of lateral subsurface flows. In the wetlands, satellite‐based ET_a estimates follow the seasonal pattern of in situ observations, but with values ~50% higher than those determined from elevation‐corrected eddy‐covariance (EC) measurements. Nonetheless, this difference is within the expected precision of the remote sensing method. Although based on a limited period, the validation results are encouraging, and demonstrate the utility of satellite tools and limited period in situ data for watersheds with scarce long‐term data.This article is categorized under: Science of Water > Hydrological Processes

The Silala is a small river, originating in the Andean Altiplano, which flows from Bolivia into Chile. Prior to a legal dispute between Chile and Bolivia over the status and use of the waters of the Silala, few hydrological studies had been performed in the basin. Further insights were required to better understand the surface‐water and groundwater discharges from Bolivia to Chile, and the effects of historical channelization of the Bolivian wetlands on these flows. A semi‐distributed hydrological model was therefore developed to estimate the discharges from the basin and provide recharge inputs to a groundwater model used to investigate the effects of channelization. Long‐term temperature and precipitation data were available for 1969–1992, while more detailed data were available for 2018–2019. 1969–1992 was selected as a suitable length of record for long‐term groundwater recharge estimation, and the recent data were reserved for model validation, reported in a companion paper. Prior model parameter ranges were identified based on field observations and scientific literature, and sampling of both input and parameter uncertainty allowed determination of representative, lower and upper groundwater recharge scenarios. Results show strong inter‐annual and seasonal variability, the largest groundwater recharge being observed during the Austral summer. A representative groundwater recharge rate of 39.5 mm/year was obtained for the basin to the international border, with feasible lower and upper bounds of 34.9 and 50.2 mm/year, respectively. This lies within the range of 21–51 mm/year estimated by Bolivia for 1969–2017, albeit higher than their best estimate (24 mm/year).This article is categorized under: Science of Water > Hydrological Processes

A dispute between Chile and Bolivia regarding the status and use of the waters of the Silala River resulted in proceedings before the International Court of Justice, initiated in 2016. The magnitude of the effect of historical channelization in Bolivia on surface water flows emerged as a major point of disagreement. Based on modeling by the Danish Hydraulic Institute (DHI), using the MIKE‐SHE and MIKE‐11 modeling systems, Bolivia suggested initially that the channels had increased surface flow by 30%–40% and later by 11%–33%. In the opinion of Chile's international experts, these effects would be small. This paper reviews the use of DHI's models by Bolivia and, subsequently, by Chile. Concerns about Bolivia's modeling raised by Chile included the selection of boundary conditions, inconsistent use of the two models for different scenarios, the use of inconsistent topographies for different scenarios, unexplained additions of water and numerical instabilities affecting the results. To investigate these discrepancies, additional simulations were performed by Chile. DHI's MIKE‐SHE and MIKE‐11 models were used for all scenarios, with consistent topography, spring and surface water flow representation. Numerical instabilities were reduced by adjustment of time steps and channel topography. These simulations showed that removing the channels reduces surface flow by a maximum of 3.5 L/s, or 2.4%. The sum of the groundwater and surface water outflow decreases by 2.6 L/s or 1.0%. This small decrease lies within the modeling error but is consistent with the expected effect of minor changes in evaporation.This article is categorized under: Science of Water > Hydrological Processes Human Water > Rights to Water Science of Water > Methods

The Weather Research and Forecasting (WRF) model has been successfully used in weather prediction, but its ability to simulate precipitation over areas with complex topography is not optimal. Consequently, WRF has problems forecasting rainfall events over Chilean mountainous terrain and foothills, where some of the main cities are located, and where intense rainfall occurs due to cutoff lows. This work analyzes an ensemble of microphysics schemes to enhance initial forecasts made by the Chilean Weather Agency in the front range of Santiago. We first tested different vertical levels resolution, land use and land surface models, as well as meteorological forcing (GFS/FNL). The final ensemble configuration considered three microphysics schemes and lead times over three rainfall events between 2015 and 2017. Cutoff low complex meteorological characteristics impede the temporal simulation of rainfall properties. With three days of lead time, WRF properly forecasts the rainiest N-hours and temperatures during the event, although more accuracy is obtained when the rainfall is caused by a meteorological frontal system. Finally, the WSM6 microphysics option had the best performance, although further analysis using other storms and locations in the area are needed to strengthen this result.