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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

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.

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.