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Effective watershed management and protection of water resources from non-point source pollution require identification, prioritization, and targeting of pollutant source areas. Process-based hydrology and water quality models are powerful heuristic tools for land and water resources managers. However, because of their complexity, such models are often under-utilized as management prioritization and planning tools. In this paper, we present a prioritization, interactive visualization, and analysis tool (Pi-VAT) that is programmed to synthesize multi-scenario, multi-watershed outputs from process-based geospatial models. We demonstrate the utility of Pi VAT to examine simulated hydrologic, sediment, and water quality response at the hillslope/hydrologic response unit (HRU) scale. We apply Pi-VAT to output from multiple watersheds and for multiple management scenarios and treatments from two geospatial models for watershed management: Water Erosion Prediction Project (WEPP) and Soil & Water Assessment Tool (SWAT). Pi-VAT was developed using the Shiny web application framework for the R programming language. In a matter of minutes, Pi-VAT can synthesize overwhelming amounts of output from process-based models into information useful for land and water resources managers. We illustrate the use of Pi-VAT to interactively identify, quantify, and visualize areas that are most susceptible to disturbance under different scenarios and provide a synthesis approach based on land use, soil type, and slope steepness. This approach guides land and water resources managers in prioritizing the areas of the watershed that provide the maximum reduction in pollutant loads while treating the least amount of area. Pi-VAT provides a flexible reactive platform for the development of decision support tools based on process-based models intended for watershed management and research applications. AFRI program [1009827]; USDA National Institute of Food and Agriculture Published version This work was supported by the AFRI program [grant no201667020-25320/project accession no. 1009827] from the USDA National Institute of Food and Agriculture. Public domain – authored by a U.S. government employee

Since the development and availability of GIS-based software and satellite imagery, there has been a vision that watershed managers would have near-real-time, three-dimensional hydrologic and soil erosion models that could easily assess impacts of watershed management decisions at high spatial resolutions across multiple scales. Our research team has made significant advances to address this challenging problem especially in the forest environment. The technology and data retrieval and access has dramatically improved to the point where it is possible to provide useful, near-real-time, geospatial decision support for watershed managers. This talk describes an online watershed model called WEPPcloud, widely used by the Forest Service and one of the FSWEPP suite of watershed tools, which is based fundamentally on a process-based hydrologic, soil erosion model (WEPP, Water Erosion Prediction Project). WEPPcloud is driven by discoverable, data-rich geospatial mapping products (e.g. soils, topography, satellite-based vegetation characteristics) and management libraries. It accesses daily grid-based historical and future projected climatic data to provide a comprehensive spatially and temporally explicit assessment of the impacts of management decisions on hydrologic response and sediment transport. Currently, WEPPcloud can be applied throughout the continental US, and beta versions are available for Australia and Europe. We will demonstrate this tools’ development and application to guide pre-fire fuel management and post-fire mitigation, flood risk for communities where drinking water supplies and water resources are vulnerable to wildfire. We will discuss the ongoing limitations, challenges and opportunities towards more fully incorporating geospatial hydrologic and soil erosion models into watershed management decisions.

In the Lake Tahoe Basin in California and Nevada (USA), managing nutrient export from watersheds into streams and the lake is a significant challenge that needs to be addressed to improve water quality. Leaching and runoff of phosphorus (P) from soils is a major nutrient source to the lake, and P loading potential from different watersheds varies as a function of landscape and ecosystem properties, and how the watershed is managed. In this research, P availability and speciation in forest and meadow soils in the Lake Tahoe Basin were measured at two watersheds with different parent material types. Soils developed on andesitic parent materials had approximately twice as much total P compared to those developed on granitic parent materials. Regardless of parent material, organic P was 79 to 92% of the total P in the meadow soils, and only 13 to 47% in the forest soils. Most of the soil organic P consisted of monoester P compounds, but a significant amount, especially in meadow soils, was diester P compounds (up to 30% of total extracted P). Water extractable P (WEP) concentrations were approximately ten times greater in the granitic forest soils compared to the andesitic forest soils, which had more poorly crystalline aluminosilicates and iron oxides that retain P and thus restrict WEP export. In the meadow soils, microbial biomass P was approximately seven times greater than the forest soils, which may be an important sink for P leached from upland forests. Results show that ecosystem and parent material are important attributes that control P speciation and availability in the Lake Tahoe Basin, and that organic P compounds are a major species of the soil P that are available for leaching from the soils. These factors can be used to develop accurate predictions of P availability and more precise forest management practices to reduce P export into Lake Tahoe.

An increased emphasis on fuel management strategies to mitigate wildfire risks is raising the awareness and need for comprehensive forest management strategies that satisfy long-term water quantity and water quality needs. Forest management activities can alter the soil nutrient pools and affect the timing and magnitude of stream water quantity and quality. We investigated the effect of contemporary forest management activities, including clear-cutting and thinning, on water yield and stream nitrogen and phosphorus dynamics in a quarter-century-long (1992–2016) paired and nested watershed study in the interior Pacific Northwest, US. Monthly water samples were collected and analyzed for total Kjeldhal nitrogen (TKN), total available nitrogen (TAN), nitrate + nitrite (NO3 + NO2), total phosphorus (TP), and orthophosphate (OP) concentrations throughout the study period. Five years of pre-disturbance data, 4 years of post-road construction, 6 years of Phase I (PH-I) experimental post-harvest, and 9 years of Phase II (PH-II) operational harvest data were analyzed using a before-after, control-impact paired series (BACIPS) study design. We found statistically significant increases in stream NO3 + NO2 loading from the paired and nested watersheds following timber harvest treatments. In the case of OP, any increase in nutrient load was attributed to increases in streamflow, as OP concentrations remained near minimum detectable concentrations. Streamflow increased the greatest following clear-cut practices (33.4% at W1 during PH-I) with the largest response in stream NO3 + NO2 concentration (up to 0.33 mg-N L−1 at W1). In-stream NO3 + NO2 and OP concentrations were lower in the downstream cumulative watersheds, which was likely due to dilution and nutrient assimilation effects. Interestingly, the NO3 + NO2 concentration, streamflow, and loads of NO3 + NO2 and OP from the undisturbed control watershed also increased. This increase was, however, smaller than the harvested watersheds and likely driven by climate variability or subtle forest succession changes. In summary, we found that contemporary forest management activities increased stream NO3 + NO2 concentrations and loads following timber harvest activities, but these effects were also attenuated due to downstream uptake processes. Furthermore, relative to post-wildfire impacts, these nutrient increases are small and short-lived.

The Water Erosion Prediction Project (WEPP) model was applied to seven paired, nested watersheds within the Mica Creek Experimental Watershed located in northern Idaho, USA. The goal was to evaluate the ability of WEPP to simulate the direct and cumulative effects of clear-cutting and partial-cutting (50% canopy removal) on water and sediment yield. WEPP was modified to better represent changes in the Leaf Area Index during post-harvest forest vegetative recovery. Good agreement between simulated and observed streamflow was achieved with minimal to no calibration over a 16-year (1992-2007) period. For the seven watersheds and the entire study period, the overall Nash-Sutcliffe Efficiency (NSE), Kling-Gupta efficiency (KGE), and deviation of runoff volume (DV) between observed and simulated daily streamflow ranged 0.58-0.71, 0.67-0.81, and -4% to 9%, respectively. Good agreement between predicted and observed suspended sediment yield was achieved through the calibration of a single channel critical shear stress parameter. For sediment yield, NSE, KGE, and DV ranged 0.62-0.97, 0.43-0.97, and -2% to 2%, respectively, for the calibration period, and 0.61-0.93, 0.42-0.95, and -24% to 13%, respectively, for the period of model performance assessment. Regression analysis of observed- and WEPP-simulated increase in water and sediment yield following clear-cut treatment was similar; however, the WEPP-simulated increase was lower compared to observations particularly from the partial-cut watershed. The variability in the critical shear parameter for different stream channels in the study watersheds was directly related to the observed mean particle size on the stream bed and suggests that applications of the WEPP model in ungauged basins could potentially set the critical shear parameter based on particle size. Overall, the simulated results demonstrate the potential of WEPP as a modeling tool for forestland watershed management, particularly for estimating the effects of forest harvest on hydrograph fluctuations and consequently, stream sediment transport.

Summary Forest managers need methods to evaluate the impacts of management at the watershed scale. The Water Erosion Prediction Project (WEPP) has the ability to model disturbed forested hillslopes, but has difficulty addressing some of the critical processes that are important at a watershed scale, including baseflow and water yield. In order to apply WEPP to forested watersheds, we developed and assessed new approaches for simulating streamflow and sediment transport from large watersheds using WEPP. We created specific algorithms to spatially distribute soil, climate, and management input files for all the subwatersheds within the basin. The model enhancements were tested on five geologically and climatically diverse watersheds in the Lake Tahoe basin, USA. The model was run with minimal calibration to assess WEPP’s ability as a physically-based model to predict streamflow and sediment delivery. The performance of the model was examined against 17 years of observed snow water equivalent depth, streamflow, and sediment load data. Only region-wide baseflow recession parameters related to the geology of the basin were calibrated with observed streamflow data. Close agreement between simulated and observed snow water equivalent, streamflow, and the distribution of fine ( 20 μm) sediments was achieved at each of the major watersheds located in the high-precipitation regions of the basin. Sediment load was adequately simulated in the drier watersheds; however, annual streamflow was overestimated. With the exception of the drier eastern region, the model demonstrated no loss in accuracy when applied without calibration to multiple watersheds across Lake Tahoe basin demonstrating the utility of the model as a management tool in gauged and ungauged basins.

Exposed mineral soil is an immediate result of forest fires with direct relevance on surface runoff and soil erosion. The goal of this study was to determine which topographic features influence the distribution of exposed mineral soil following wildfire in forested watersheds. In a field investigation 2 months after a simulated wildfire, ground cover was measured and a number of topographic variables, including slope, aspect, distance to streams, solar radiation, elevation, curvature, and profile curvature, were used to determine their relationships to postfire exposed mineral soil. The spatial distribution of fire effects along hill slopes was assessed using geographic information system (GIS), and a regressional relationship was derived to estimate postfire exposed mineral soil from key topographic variables, namely, aspect, solar radiation, and profile curvature. Solar radiation, aspect, and profile curvature were significantly correlated with the amount of soil exposed after the fire; however, aspect and profile curvature were the leading variables in the predictive regression model. Convex-shaped profile curvatures on south-facing slopes had higher values of exposed mineral soil than concave surfaces on north-facing slopes. These results can help land managers with decisions on prioritizing areas in need of pre- or postfire treatment.

The assessment of water yield from hillslopes into streams is critical in managing water supply and aquatic habitat. Streamflow is typically composed of surface runoff, subsurface lateral flow, and groundwater baseflow; baseflow sustains the stream during the dry season. The Water Erosion Prediction Project (WEPP) model simulates surface runoff, subsurface lateral flow, soil water, and deep percolation. However, to adequately simulate hydrologic conditions with significant quantities of groundwater flow into streams, a baseflow component for WEPP is needed. The objectives of this study were (1) to simulate streamflow in the Priest River Experimental Forest in the U.S. Pacific Northwest using the WEPP model and a baseflow routine, and (2) to compare the performance of the WEPP model with and without including the baseflow using observed streamflow data. The baseflow was determined using a linear reservoir model. The WEPP-simulated and observed streamflows were in reasonable agreement when baseflow was considered, with an overall Nash-Sutcliffe efficiency (NSE) of 0.67 and deviation of runoff volume (D v ) of 7%. In contrast, the WEPP simulations without including baseflow resulted in an overall NSE of 0.57 and D v of 47%. On average, the simulated baseflow accounted for 43% of the streamflow and 12% of precipitation annually. Integration of WEPP with a baseflow routine improved the model’s applicability to watersheds where groundwater contributes to streamflow.

Assessment of water yields from watersheds into streams and rivers is critical to managing water supply and supporting aquatic life. Surface runoff typically contributes the most to peak discharge of a hydrograph while subsurface flow dominates the falling limb of hydrograph and baseflow contributes to streamflow from shallow unconfined aquifers primarily during the non-rainy season. The Water Erosion Prediction Project (WEPP) model is a physically-based, distributed-parameter, continuous-simulation model. Recent improvements to WEPP include enhanced computation of evapotranspiration (ET) by incorporating the Penman-Monteith method into the model, and improved calculation of subsurface lateral flow by properly setting a restrictive layer and soil anisotropic ratios. These modifications have substantially improved the performance of the WEPP model for forested watersheds. In order to further enhance the model applicability, a baseflow component needs to be incorporated to adequately represent hydrologic conditions where significant quantities of ground water flow to streams.