Precipitation is a key input variable in distributed surface water‐groundwater models, and its spatial variability is expected to impact watershed hydrologic response via changes in subsurface flow ...dynamics. Gridded precipitation data sets based on gauge observations, however, are plagued by uncertainty, especially in mountainous terrain where gauge networks are sparse. To examine the mechanisms via which uncertainty in precipitation data propagates through a watershed, we perform a series of numerical experiments using an integrated surface water‐groundwater hydrologic model, ParFlow.CLM. The Kaweah River watershed in California, USA, is used as our virtual catchment laboratory to characterize watershed response to variable precipitation forcing from headwaters to groundwaters. By applying the three‐cornered hat method, we quantify the spatially distributed uncertainty in four publically available precipitation forcing data sets and their simulated hydrology. Simulations demonstrate that uncertainty in the simulated groundwater storage is primarily a result of topographic redistribution of uncertainty in precipitation forcing. Soil water redistribution is the primary pathway that redistributes uncertainty downslope. We also find that topography exerts a larger impact than variable subsurface parameters on propagating uncertainty in simulated fluxes. Finally, we find that improvement in model performance metrics is higher for a single simulation forced with the mean precipitation from the available data sets than the averaged simulated results of separate simulations forced with each data set. Results from this study highlight the importance of topography‐moderated flow through the critical zone in shaping the groundwater response to climate variability.
Key Points
Uncertainty in simulated groundwater storage change is a result of topographic redistribution of uncertainty in precipitation forcing
Subsurface flow pathways in both soil and deeper bedrock control watershed response to variable precipitation input
Merging multiple precipitation data sets provides better results than merging simulated fluxes, due to high model sensitivity to changes in precipitation
Mountainous regions act as the water towers of the world
by producing streamflow and groundwater recharge, a function that is
particularly important in semiarid regions. Quantifying rates of mountain
...system recharge is difficult, and hydrologic models offer a method to
estimate recharge over large scales. These recharge estimates are prone to
uncertainty from various sources including model structure and parameters.
The quality of meteorological forcing datasets, particularly in mountainous
regions, is a large source of uncertainty that is often neglected in
groundwater investigations. In this contribution, we quantify the impact of
uncertainty in both precipitation and air temperature forcing datasets on
the simulated groundwater recharge in the mountainous watershed of the
Kaweah River in California, USA. We make use of the integrated surface water–groundwater model, ParFlow.CLM, and several gridded datasets commonly used
in hydrologic studies, downscaled NLDAS-2, PRISM, Daymet, Gridmet, and
TopoWx. Simulations indicate that, across all forcing datasets, mountain front recharge is an important component of the water budget in the
mountainous watershed, accounting for 9 %–72 % of the annual precipitation and ∼90 % of the total mountain system recharge to the
adjacent Central Valley aquifer. The uncertainty in gridded air temperature
or precipitation datasets, when assessed individually, results in similar
ranges of uncertainty in the simulated water budget. Variations in simulated
recharge to changes in precipitation (elasticities) and air temperature
(sensitivities) are larger than 1 % change in recharge per 1 % change in
precipitation or 1 ∘C change in temperature. The total volume of
snowmelt is the primary factor creating the high water budget sensitivity, and snowmelt volume is influenced by both precipitation and air temperature
forcings. The combined effect of uncertainty in air temperature and
precipitation on recharge is additive and results in uncertainty levels roughly equal to the sum of the individual uncertainties depending on the
hydroclimatic condition of the watershed. Mountain system recharge pathways
including mountain block recharge, mountain aquifer recharge, and mountain
front recharge are less sensitive to changes in air temperature than changes
in precipitation. Mountain front and mountain block recharge are more
sensitive to changes in precipitation than other recharge pathways. The
magnitude of uncertainty in the simulated water budget reflects the
importance of developing high-quality meteorological forcing datasets in mountainous regions.
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.
Key Points
A distributed hydrologic model is tested with long‐term measurements of water and energy states and fluxes in a piedmont slope watershed
When modified to account for transient channel losses, the model reproduces well the percolation estimates and observed streamflow response
Modeling scenarios reveal the relative importance of hillslope and channel properties on runoff generation and percolation losses
Water returned to the atmosphere as evapotranspiration (ET) is approximately 1.6x global river discharge and has wide‐reaching impacts on groundwater and streamflow. In the U.S. Midwest, widespread ...land conversion from prairie to pasture to cropland has altered spatiotemporal patterns of ET, yet there is not consensus on the direction of change or the mechanisms controlling changes. We measured ET at three locations within the Long‐Term Agroecosystem Research network along a latitudinal gradient with paired rainfed cropland and prairie sites at each location. At the northern locations, the Upper Mississippi River Basin (UMRB) and Kellogg Biological Station (KBS), the cropland has annual ET that is 84 and 29 mm/year (22% and 5%) higher, respectively, caused primarily by higher ET during springtime when fields are fallow. At the southern location, the Central Mississippi River Basin (CMRB), the prairie has 69 mm/year (11%) higher ET, primarily due to a longer growing season. Differences in climate and that the CMRB prairie is remnant native prairie, while the UMRB and KBS prairies are restored, make it challenging to attribute differences to specific mechanisms. To accomplish this, we examine the energy balance using the Two‐Resistance Method (TRM). Results from the TRM demonstrate that higher surface conductance in croplands is the primary factor leading to higher springtime ET from croplands, relative to prairies. Results from this study provide insight into impacts of warm season grasses on the hydrology of the U.S. Corn Belt by providing a mechanistic understanding of how land use change affects the water budget.
Plain Language Summary
Evapotranspiration (ET) consists of evaporation from bare soil and plant leaves. ET is ∼1.6x greater than global river flow and has wide‐reaching impacts on groundwater and streamflow. In the U.S. Midwest, widespread land conversion from prairies to croplands has altered patterns of ET, yet there is no consensus on the direction of this change or the mechanisms controlling changes. In this study we use measurements of ET at three locations within the Long‐Term Agroecosystem Research (LTAR) network that have paired cropland and prairie sites. Surprisingly, we found that in the two northern sites, the croplands had higher ET than the prairies, particularly during springtime when the croplands are fallow. We used mathematical analysis of the energy budget to show that a parameter called the surface conductance controls the differences in ET between the croplands and prairies. During springtime in prairies, the standing, dormant vegetation blocks transfer of water vapor from the land surface, reducing the surface conductance, and limits the ET. Results from this study provide insight into the impact of land conversion from prairies to croplands on the hydrology of the U.S. Corn Belt by providing a mechanistic understanding of how land use change affects the water budget.
Key Points
Differences in evapotranspiration between croplands and prairies was quantified by a mechanistic Two Resistance Method
Bowen ratio during springtime is higher in prairies than croplands
Surface resistance is the primary factor causing springtime evapotranspiration differences between croplands and prairies
Accurate simulation of plant water use across agricultural ecosystems is essential for various applications, including precision agriculture, quantifying groundwater recharge, and optimizing ...irrigation rates. Previous approaches to integrating plant water use data into hydrologic models have relied on evapotranspiration (ET) observations. Recently, the flux variance similarity approach has been developed to partition ET to transpiration (T) and evaporation, providing an opportunity to use T data to parameterize models. To explore the value of T/ET data in improving hydrologic model performance, we examined multiple approaches to incorporate these observations for vegetation parameterization. We used ET observations from five eddy covariance towers located in the irrigated San Joaquin Valley, California, to parameterize orchard crops in an integrated land surface—groundwater model. By using ET, or both ET and T data, we examined the impact of multiple model parameterization approaches ranging from simple performance metrics to the generalized likelihood uncertainty estimation method. We find that a simple approach of selecting the parameter sets based on ET and T performance metrics works best at these study sites. Selecting parameters based on performance relative to observed ET creates an uncertainty of 27% relative to the observed value. When parameters are selected using both T and ET data, this uncertainty drops to 24%. Similarly, the uncertainty in potential groundwater recharge drops from 63% to 58% when parameters are selected with ET or T and ET data, respectively. While these improvements are minor in an irrigated setting, the value of partitioning ET data may be more useful in non‐irrigated settings. Additionally, using crop type parameters results in similar levels of simulated ET as using site‐specific parameters. Different irrigation schemes create high amounts of uncertainty and highlight the need for accurate estimates of irrigation when performing water budget studies.
Accurate simulation of plant water use across agricultural ecosystems is essential for various applications, including precision agriculture, quantifying groundwater recharge and optimizing irrigation rates. We use eddy covariance measurements and partition evapotranspiration into plant water use and bare soil evaporation to improve parameterization of vegetation in a land surface model. We test approaches to incorporate this data and quantify the uncertainty in the simulated water budget.
Agricultural production in highly variable soils is a challenge, especially when those soils are shallow. Precision agriculture techniques were developed to improve yields and minimize spatial and ...interannual variability in profit. In the Central Claypan region of the Midwest United States, many of the precision agriculture techniques were based on the assumption that topsoil depth controlled plant available water, and therefore yield. But this assumption has not been empirically tested. In this study, we use measurements of sap flow installed on maize plants with a gradient in topsoil depth, caused by a claypan layer. We hypothesize that plants with higher water use have higher yield, plants in areas with thicker topsoil have higher water use, and soils in the areas with thicker topsoil have higher soil water content. Sap flow sensors were installed on 5 plants each at three locations with topsoil depths of 19.6cm (shallow), 21.6cm (medium), and 30.5cm (deep) from June – September, 2022. An ANOVA analysis demonstrates that the average total season transpiration at the deep site (279mm) was significantly larger than at the shallow site (151mm), while the medium site was in the middle with transpiration not significantly different from either shallow or deep sites (218mm). At the end of the season, the plants were harvested and total biomass and grain yield were measured. Increase in plant transpiration was significantly related to both increases in biomass and yield. Finally, we measured volumetric soil water content at each location and found higher soil water content at the site with thicker topsoil. Our results demonstrate the link between topsoil depth, soil water content, plant transpiration, and yield. These findings will help improve precision agriculture techniques in areas with highly variable topsoil thickness.
•Increased transpiration is related to increased plant biomass and grain yield.•Sites with deeper topsoil have higher plant available water for a growing season.•Higher soil water content is correlated with higher plant transpiration.
A limited understanding of how extreme weather events affect groundwater hinders our ability to predict climate change impacts in drylands, where channel transmission losses are often the primary ...recharge mechanism. In this study, we investigate how potential changes to precipitation intensity and temperature will affect the water balance of a typical first-order, arid watershed located in the Chihuahuan Desert. We utilize a process-based hydrologic model driven by stochastically-downscaled simulations from a set of climate models, emissions scenarios, and future periods. Across many simulations, the average daily storm size is the primary factor that controls transmission losses with larger precipitation amounts increasing channel infiltration while simultaneously decreasing land surface evapotranspiration. Extreme events (>25 mm d−1) that account for less than 30% of the annual precipitation, contribute almost 50% of the focused recharge. As a result, climatic changes leading to larger, less frequent storms will result in higher channel transmission losses in arid regions.
Woody plant encroachment is a global phenomenon whereby shrubs or trees replace grasses. The hydrological consequences of this ecological shift are of broad interest in ecohydrology, yet little is ...known of how plant and intercanopy patch dynamics, distributions, and connectivity influence catchment‐scale responses. To address this gap, we established research catchments in the Sonoran and Chihuahuan Deserts (near Green Valley, Arizona and near Las Cruces, New Mexico, respectively) that represent shrub encroachment in contrasting arid climates. Our main goals in the coordinated observations were to: (a) independently measure the components of the catchment water balance, (b) deploy sensors to quantify the spatial patterns of ecohydrological processes, (c) use novel methods for characterizing catchment properties, and (d) assess shrub encroachment impacts on ecohydrological processes through modelling studies. Datasets on meteorological variables; energy, radiation, and CO2 fluxes; evapotranspiration; soil moisture and temperature; and runoff at various scales now extend to nearly 10 years of observations at each site, including both wet and dry periods. Here, we provide a brief overview of data collection efforts and offer suggestions for how the coordinated datasets can be exploited for ecohydrological inferences and modelling studies. Given the representative nature of the catchments, the available databases can be used to generalize findings to other catchments in desert landscapes.
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.
Woody plant encroachment (WPE) into grasslands is a global phenomenon that is associated with land degradation via xerification, which replaces grasses with shrubs and bare soil patches. It remains ...uncertain how the global processes of WPE and climate change may combine to impact water availability for ecosystems. Using a process-based model constrained by watershed observations, our results suggest that both xerification and climate change augment groundwater recharge by increasing channel transmission losses at the expense of plant available water. Conversion from grasslands to shrublands without creating additional bare soil, however, reduces transmission losses. Model simulations considering both WPE and climate change are used to assess their relative roles in a late 21
century condition. Results indicate that changes in focused channel recharge are determined primarily by the WPE pathway. As a result, WPE should be given consideration when assessing the vulnerability of groundwater aquifers to climate change.
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