Abstract. High-resolution characterizations and predictions are a grand challenge for ecohydrology.Recent advances in flight control, robotics and miniaturized sensors using unmanned aerial vehicles (UAVs) provide an unprecedented opportunity for characterizing, monitoring and modeling ecohydrologic systems at high-resolution (,1 m) over a range of scales. How can the ecologic and hydrologic communities most effectively use UAVs for advancing the state of the art? This Innovative Viewpoints paper introduces the utility of two classes of UAVs for ecohydrologic investigations in two semiarid rangelands of the southwestern U.S. through two useful examples. We discuss the UAV deployments, the derived image, terrain and vegetation products and their usefulness for ecohydrologic studies at two different scales. Within a land-atmosphere interaction study, we utilize high-resolution imagery products from a rotarywing UAV to characterize an eddy covariance footprint and scale up environmental sensor network observations to match the time-varying sampling area. Subsequently, in a surface and subsurface interaction study within a small watershed, we demonstrate the use of a fixed-wing UAV to characterize the spatial distribution of terrain attributes and vegetation conditions which serve as input to a distributed ecohydrologic model whose predictions compared well with an environmental sensor network. We also point to several challenges in performing ecohydrology with UAVs with the intent of promoting this new self-service (do-it-yourself ) model for high-resolution image acquisition over many scales. We believe unmanned aerial vehicles can fundamentally change how ecohydrologic science is conducted and offer ways to merge remote sensing, environmental sensor networks and numerical models.

Abstract. Soil moisture dynamics reflect the complex interactions of meteorological conditions with soil, vegetation and terrain properties. In this study, intermediate-scale soil moisture estimates from the cosmic-ray neutron sensing (CRNS) method are evaluated for two semiarid ecosystems in the southwestern United States: a mesquite savanna at the Santa Rita Experimental Range (SRER) and a mixed shrubland at the Jornada Experimental Range (JER). Evaluations of the CRNS method are performed for small watersheds instrumented with a distributed sensor network consisting of soil moisture sensor profiles, an eddy covariance tower, and runoff flumes used to close the water balance. We found a very good agreement between the CRNS method and the distributed sensor network (root mean square error (RMSE) of 0.009 and 0.013 m 3 m −3 at SRER and JER, respectively) at the hourly timescale over the 19-month study period, primarily due to the inclusion of 5 cm observations of shallow soil moisture. Good agreement was also obtained in soil moisture changes estimated from the CRNS and watershed water balance methods (RMSE of 0.001 and 0.082 m 3 m −3 at SRER and JER, respectively), with deviations due to bypassing of the CRNS measurement depth during large rainfall events. Once validated, the CRNS soil moisture estimates were used to investigate hydrological processes at the footprint scale at each site. Through the computation of the water balance, we showed that drier-than-average conditions at SRER promoted plant water uptake from deeper soil layers, while the wetter-than-average period at JER resulted in percolation towards deeper soils. The CRNS measurements were then used to quantify the link between evapotranspiration and soil moisture at a commensurate scale, finding similar predictive relations at both sites that are applicable to other semiarid ecosystems in the southwestern US.

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Abstract. A critical hydrologic process in arid and semiarid regions is the interaction between ephemeral channels and groundwater aquifers. Generally, it has been found that ephemeral channels contribute to groundwater recharge when streamflow infiltrates into the sandy bottoms of channels. This process has traditionally been studied in channels that drain large areas (tens to hundreds of square kilometers). Since the water table in arid and semiarid regions is typically far from the surface, measured streamflow losses or percolation into the deep vadose zone is equated to groundwater recharge. In this study, we use a water balance approach to estimate deep percolation in a first-order, instrumented watershed (4.7 ha) on a piedmont slope of the Jornada Experimental Range (JER) in the Chihuahuan Desert. Results indicate that runoff generated within the piedmont slope contributes significantly to deep percolation. During the short-term 6-yr study period, we estimated 385 mm of total percolation, 62 mm/yr, or a ratio of percolation to rainfall of 0.26. Based on the instrument network, we identified that percolation occurs inside channel areas when these receive overland sheetflow from hillslopes. We observed less streamflow leaving the watershed as compared to percolation during the study period, leading to an outlet streamflow to rainfall ratio of 0.02. Using long-term data sets available from the JER, we estimate that over the last 100 yr, 48 mm/yr of percolation occurs at the study site, a ratio of percolation to rainfall of 0.19. We then scale this up to determine the contribution of similarly sized watersheds on the piedmont slopes of the JER. These observations highlight the importance of arid piedmont slopes for generating groundwater recharge, in particular during above-average rainfall periods.

Channel transmission losses alter the streamflow response of arid and semiarid watersheds and promote focused groundwater recharge. This process has been primarily studied in dryland channels draining large areas that are displaced away from hillslope runoff generation. In contrast, small watersheds on arid piedmont slopes allow the investigation of interactive hillslope and channel processes that control the partitioning between surface and subsurface flows. In this study, we utilize high‐resolution, long‐term measurements of water balance components in an instrumented watershed of the Chihuahuan Desert to set up, parameterize, and test a process‐based, distributed hydrologic model modified to account for channel losses. A transient method for capturing capillary effects in channels results in simulations with a reliable representation of the watershed energy balance, soil moisture dynamics, hillslope infiltration, channel transmission (or percolation) losses, and streamflow yield over the study period. The simulation also reproduces a conceptual model of hillslope infiltration‐excess runoff generation linked to downstream channel percolation losses that depend on the rainfall event size. Model‐derived thresholds were obtained for the amount of hillslope runoff (6 mm) and rainfall (12.5 mm) necessary for streamflow yield, such that 40% of percolation occurs for small events that do not reach the outlet. Using a set of scenarios, we identify that hillslope infiltration controls the rainfall threshold necessary to initiate percolation, while channel infiltration affects the partitioning into percolation and streamflow yield. Thus, the connectivity along hillslope‐channel pathways is deemed an essential control on the streamflow generation and groundwater recharge in arid regions with complex terrain.