RSS FeedsSurface Water Hydrology
Integrated Surface Water Model
Model Development
Model Calibration
Results
The surface water hydrology chapter describes the baseline conditions for surface water hydrology of Napa County. This chapter is one of three chapters related to the hydrological system of the County. The hydrological system represents the occurrence, movement, and distribution of water movement in the air, on the ground, and beneath the surface.
The three hydrology/water-related chapters of the BDR characterize surface water, groundwater, and water quality conditions. These chapters also describe developing a regionally integrated surface water, groundwater, and water quality models developed for Napa County. It is important to note that the three-part surface water hydrology, groundwater, and water quality analyses and models were conducted and developed with the understanding that such models and analyses would be applied towards future planning considerations. More specifically, the hydrology studies were designed to establish baseline conditions by which Countywide projects and programs could be assessed and evaluated for their planning benefits, constraints, and environmental impacts.
Prior to building a model of the Napa County hydrologic system, the main features and driving forces of the natural hydrologic system were identified based on existing information, past studies, field visits, and engineering hydrology judgment. A conceptual model was then developed to describe hydrologic functioning, identify the significant hydrologic variables needed in the model, and provide a basis to advance the analysis and develop a valid mathematical model.
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INTEGRATED SURFACE WATER MODEL
The integrated surface water, groundwater, and surface water quality models developed for the Napa BDR project are based on the MIKE SHE/MIKE11 code developed by DHI Water and Environment. The MIKE SHE/MIKE11 code is a physically based, distributed hydrologic model that simulates the major flow components of the hydrologic cycle, making it very well suited for simulating current and future water distribution in Napa County. Results from the surface water model include monthly values, graphs, and maps for all the major flow components of the hydrologic cycle; stream hydrographs; and water surface profiles. These models are also scalable, which allows for regional modeling as well as with more spatially detailed data, examination of the local effects of a project on the hydrologic system.
The principal flow components in Napa County’s hydrological system include precipitation, evapotranspiration, overland flow, surface water runoff, vadose zone flow, and groundwater recharge. These components were each accounted for in developing the MIKE SHE/MIKE11 hydrology model of Napa County.
MODEL DEVELOPMENT
Drainage Network, Subbasins and Streamflow
Independent models were developed for the hydrologically separate Napa River, Putah Creek, and Suisun Creek watersheds Each basin was further subdivided into a total of 188 subbasins to provide the County with a comprehensive set of subbasin planning units that could each be evaluated for baseline and alternative conditions. Delineation of the subbasin boundaries was based on Digital Elevation Maps (DEMs) of the County. Napa County covers approximately 728 m.2 and the model uses a grid size of 250 m. For purposes of flow routing, a principal drainage in each of the 188 subbasins was used for downstream routing.
Precipitation and Evapotranspiration
Precipitation distribution in Napa County is influenced by regional weather patterns as well as local county microclimates. In the Napa River Watershed, 69 precipitation polygons were linked with precipitation records to simulate the historic distribution of the precipitation. Evapotranspiration for all models used the rates from the CIMIS Oakville, Carneros, and Angwin stations.
Land Cover
For modeling land cover, vegetation types described in the Biologic Resources Chapter of the BDR were used to describe bare ground, coniferous forest, deciduous shrubs, deciduous woodland, developed, Eucalyptus woodland, evergreen broadleaf wood, evergreen scrubland, grassland, rock outcrop, unclassified, vineyard, water, and wetland land covers. Parameters for rooting depth, leaf area index, overland flow roughness, and evapotranspiration were determined for each vegetation class.
Soil Water & Groundwater
Simulation of groundwater used a two-layer water balance to represent the shallower vadose zone and a series of linear reservoirs to simulate deeper interflow and base flow zones. The Natural Resource Conservation Service’s (NRCS) Soil Survey Geographic Database (SSURGO) was used as the soil base data including depth, saturation conductivity, saturation point, wilting point, and field capacity. Napa County general soil types include: clay loam, deep clay, deep fine loam/clay, gravelly loam, loam, rock, undeep loam, and undeep loam/gravel. As part of the Groundwater Resources analysis (Chapter 16 of the BDR) a more complete physically based integrated surface water-groundwater model was developed for areas of the County that currently use significant portions of groundwater.
Water Use
In the northern Napa River Watershed, the majority of water demand is for agricultural use (about 80%), with 12% for municipal and industrial use, and 8% for other rural use. Water sources include: 60% from groundwater, 26% from diversion of water from the Napa River, 11% from municipal reservoirs, 1% from water imported from outside the watershed, and 1% from reclaimed water. In contrast, in the southern portion of the watershed, including the City of Napa, the majority of demand is for municipal and industrial use (about 65%), with 28% for agricultural use, and 7% for rural use. In the southern watershed, water sources include: 31% from municipal and other reservoirs, 29% from water imported from outside the watershed, 26% from groundwater sources, 7% from diversion of water from the Napa River and other rivers, and 7% from reclaimed water. In the Lake Berryessa and Suisun Creek Watersheds about 98% of water demand is for agricultural use, with 2% for rural use, with 46% coming from groundwater, 21% from Lake Berryessa and other reservoirs, 19% from diversion of water from the Napa River and other rivers, 11% from water imported from outside the watershed, and 3% from reclaimed water. In 2001, 34,900 acres of agricultural lands were under irrigation in the Napa River Watershed and 2,600 acres of agricultural lands were under irrigation in the Lake Berryessa and Suisun Creek Watersheds. This agriculture is almost entirely (96-98%) vineyards and the remaining lands are pastures and lands producing other truck crops. Irrigation applied water was 38,600 ac-ft for vineyards, 1,500 ac-ft for pastures, and 700 ac-ft for other truck crop areas in the Napa River Watershed during 2001.
MODEL CALIBRATION
| Groundwater hydrologic analyses and modeling conducted in support of the BDR were undertaken with the intention of applying the models and analyses for future planning. |
Model calibration compares model results with actual streamflow data. In this way, model results are directly evaluated to see if “too much” or “too little” streamflow resulted from the modeling simulation, compared to how observed precipitation events resulted in streamflow. Stream gauge data is available along nine stations along the Napa River system in the western study area, as well as at two locations along Putah Creek: one downstream of Monticello Dam and the study area and the other upstream of Lake Berryessa and the study area. Additional historical (1961–1980) discharge data is available at five locations within the study area.
RESULTS
The Napa County MIKE SHE/MIKE 11 model is a dynamic model that can be refined and expanded as data becomes available and as new questions are identified. As the model is currently set up for regional analysis of the Napa County hydrologic system, it can be used to help evaluate alternatives developed as part of the current updating of the Napa County General Plan. In this way, the model can also be used to support a countywide program Environmental Impact Report of the General Plan Update (including evaluation of cumulative impacts). As described above, with adequate local data, the baseline model can also be developed for more localized and site specific environmental analyses of specific projects. In turn, the development of local information for site-specific projects can then be “returned” or input into the broader countywide model to also improve the accuracy of the regional model. A more complete description of the surface hydrology model and its results are found in a separate hydrology report (Napa BDR Surface Hydrology Modeling Report).
The model will be suitable for these purposes and provide a basis to compare alternatives and evaluate environmental impacts.
