No abstract

Erosion and runoff have been observed to increase following fire. Land managers and Burned Area Emergency Rehabilitation (BAER) teams must be able to estimate these post-fire changes. Studies of post-fire erosion on burned watersheds show that the concentrations of sediment eroded from burned rangeland and forested hillslopes in the southwestern United States can be extremely high. Since wildfire primarily impacts soils and vegetation cover on hillslopes, it is appropriate to assume that changes in hillslope conditions will result in changes in runoff peak, volume and sediment yield. The AGWA (Automated Geospatial Watershed Assessment www.tucson.ars.ag.gov/agwa) hydrologic modeling tool employs both an empirical model (SWAT) and a more process-based model (KINEROS2). In order to study how these models should be modified to provide land managers with a means to assess the impact of fire, the models were applied on two burned watersheds. Analysis of data from the Marshall Gulch watershed near Tucson, Arizona, indicates that changes in runoff volume are small compared to changes in peak runoff. The application of the KINEROS2 model to burned conditions at the Starmer Canyon near Los Alamos, New Mexico shows a pattern of change over time that is consistent with watershed recovery. Calibrated hillslope roughness values are consistent with independent estimates for roughness under bare conditions following the fire to roughness consistent with forested conditions three years later. The modeling also indicated that increasing hillslope roughness over time accounts for much of the change in runoff response. Some of the physical changes following fire that have been identified to contribute to changes in hydrologic response include (DeBano et al. 1998):

Appropriate land‐use, watershed‐management, and flood‐attenuation plans are critical in the cross‐border urban environment known collectively as Ambos Nogales. This paper summarizes methodologies for predicting the watershed response associated with land‐use change within a spatial and temporal context through the use of a hydrological model in a cross‐border setting. The KINEROS2 model is implemented via the Automated Geospatial Watershed Assessment 2.0 geographic information system interface to evaluate the watershed of Nogales, Arizona, and Nogales, Sonora, Mexico, to assess flood vulnerability by quantifying volumes of runoff and peak flow, based on alternative land‐use scenarios. Cross‐border geospatial data acquisition and input to models are described. Discussions about the KINEROS2 model results identify flood‐prone areas, simulate the impact of land‐use change, and evaluate the impact of potential flood‐control interventions in the Ambos Nogales watershed. Products from this research are being used in a comprehensive plan for sustainable development in Ambos Nogales.

There is a global need for management of river flows to be informed by science to protect and restore biodiversity and ecological function while maintaining water supply for human needs. However, a lack of data at large scales presents a substantial challenge to developing a scientifically robust approach to flow management that can be applied at a basin and valley scale. In most large systems, only a small number of aquatic ecosystems have been well enough studied to reliably describe their environmental water requirements. The umbrella environmental asset (UEA) approach uses environmental water requirements developed for information-rich areas to represent the water requirements of a broader river reach or valley. We illustrate this approach in the Murray-Darling Basin (MDB) in eastern Australia, which was recently subject to a substantial revision of water management arrangements. The MDB is more than 1 million km 2 with 18 main river valleys and many thousands of aquatic ecosystems. Detailed eco-hydrologic assessments of environmental water requirements that focused on the overbank, bankfull and fresh components of the flow regime were undertaken at a total of 24 UEA sites across the MDB. Flow needs (e.g. flow magnitude, duration, frequency and timing) were established for each UEA to meet the needs of key ecosystem components (e.g. vegetation, birds and fish). Those flow needs were then combined with other analyses to determine sustainable diversion limits across the basin. The UEA approach to identifying environmental water requirements is a robust, science-based and fit-for-purpose approach to determining water requirements for large river basins in the absence of complete ecological knowledge.

Rapid post-fire watershed assessment to identify potential trouble spots for erosion and flooding can potentially aid land managers and Burned Area Emergency Rehabilitation (BAER) teams in deploying mitigation and rehabilitation resources. These decisions are inherently complex and spatial in nature and require a distributed hydrological modeling approach. The extensive data requirements and the task of building input parameter files have presented obstacles to the timely and effective use of complex distributed rainfallrunoff and erosion models by BAER teams and resource managers. Geospatial tools and readily-available digital sources of pre-fire land cover, topography, and soils combined with rainfall-runoff and erosion models can expedite assessments if properly combined, provided a post-fire burn-severity map is available. The AGWA (Automated Geospatial Watershed Assessment) hydrologic modeling tool was developed to utilize nationally available spatial data sets and both empirical (SWAT) and more process-based (KINEROS2) distributed hydrologic models (see: www.tucson.ars.ag.gov/agwa). Through an intuitive interface the user selects an outlet from which AGWA delineates and discretizes the watershed using a Digital Elevation Model (DEM). The watershed model elements are then intersected with soils and land cover data layers to derive the requisite model input parameters. The chosen model is then run, and the results are imported back into AGWA for graphical display. AGWA can difference results from pre-and post-fire model simulations and display the change on the modeled watershed. This allows managers to identify potential problem areas where mitigation activities can be focused. An overview of AGWA and an application of it to the 2003 Aspen fire north of Tucson, Arizona are discussed herein.

The hydrological consequences of wildfires are among their most significant and long-lasting effects. As wildfire severity affects post-fire hydrological response, fuel treatments can be a useful tool for land managers to moderate this response. However, current models focus on only one aspect of the fire–watershed linkage (fuel treatments, fire behaviour, fire severity, watershed responses). This study outlines a spatial modelling approach that couples three models used sequentially to allow managers to model the effects of fuel treatments on post-fire hydrological responses. Case studies involving a planned prescribed fire at Zion National Park and a planned mechanical thinning at Bryce Canyon National Park were used to demonstrate the approach. Fuel treatments were modelled using FuelCalc and FlamMap within the Wildland Fire Assessment Tool (WFAT). The First Order Fire Effects Model (FOFEM) within WFAT was then used to evaluate the effectiveness of the fuel treatments by modelling wildfires on both treated and untreated landscapes. Post-wildfire hydrological response was then modelled using KINEROS2 within the Automated Geospatial Watershed Assessment tool (AGWA). This coupled model approach could help managers estimate the effect of planned fuel treatments on wildfire severity and post-wildfire runoff or erosion, and compare various fuel treatment scenarios to optimise resources and maximise mitigation results.

Urban hydrology and green infrastructure (GI) can be modeled using the Automated Geospatial Watershed Assessment (AGWA) Urban tool and the Kinematic Runoff and Erosion (KINEROS2) model. The KINEROS2 model provides an urban modeling element with nine overland flow components that can be used to represent various land cover types commonly found in the built environment while treating runoff-runon and infiltration processes in a physically based manner. The AGWA Urban tool utilizes a Geographic Information System (GIS) framework to prepare parameters required for KINEROS2, executes the model, and imports results for visualization in the GIS. The AGWA Urban tool was validated on a residential subdivision in Arizona, USA, using 47 rainfall events (June 2005 to September 2006) to compare observed runoff volumes and peak flow rates with simulated results. Comparison of simulated and observed runoff volumes resulted in a slope of 1.00 for the regression equation with an R2 value of 0.80. Comparison of observed and simulated peak flows had a slope of 1.12 with an R2 value of 0.83. A roof runoff analysis was simulated for 787 events, from January 2006 through December 2015, to analyze the water availability from roof runoff capture. Simulation results indicated a 15% capture of the average monthly rainfall volume on the watershed. Additionally, rainwater captured from roofs has the potential to provide for up to 70% of the domestic annual per capita water use in this region. Five different scenarios (S1 - base, S2 - with retention basins, S3 - with permeable driveways, S4 - with rainwater harvesting cisterns, and S5 - all GI practices from S2, S3, and S4) were simulated over the same period to compare the effectiveness of GI implementation at the parcel level on runoff and peak flows at the watershed outlet. Simulation results indicate a higher runoff volume reduction for S2 (53.41 m3 average capacity, average 30% reduction) as compared to S3 (average 14% reduction), or S4 (3.78 m3 capacity, average 6% reduction). Analysis of peak flows reveal larger peak flow reduction for S2. S3 showed more reduction of smaller peak flows as compared to S4.

Representation of precipitation is one of the most difficult aspects of modelling post-fire runoff and erosion and also one of the most sensitive input parameters to rainfall-runoff models. The impact of post-fire convective rainstorms, especially in semiarid watersheds, depends on the overlap between locations of high-intensity rainfall and areas of high-severity burns. One of the most useful applications of models in post-fire situations is risk assessment to quantify peak flow and identify areas at high risk of flooding and erosion. This study used the KINEROS2/AGWA model to compare several spatial and temporal rainfall representations of post-fire rainfall-runoff events to determine the effect of differing representations on modelled peak flow and determine at-risk locations within a watershed. Post-fire rainfall-runoff events at Zion National Park in Utah and Bandelier National Monument in New Mexico were modelled. Representations considered included both uniform and Soil Conservation Service Type II hyetographs, applying rain over the entire watershed and applying rain only on the burned area, and varying rainfall both temporally and spatially according to radar data. Results showed that rainfall representation greatly affected modelled peak flow, but did not significantly alter the model’s predictions for high-risk locations. This has important implications for post-fire assessments before a flood-inducing rainfall event, or for post-storm assessments in areas with low-gauge density or lack of radar data due to mountain beam blockage.