UMRWA
WARMF Model
This watershed hydrologic simulation model was developed as a key element of UMRWA’s
Upper Mokelumne River Watershed Assessment and Planning Project. The watershed
analysis and risk management framework model (WARMF) was used to analyze existing
hydrologic and water quality characteristics in the entire Upper Mokelumne River
watershed. It also modeled how water quality conditions could be affected based on
changes to land uses and activities. The model facilitated a better understanding of
watershed processes, which in turn enabled the Project Advisory Committee to explore
potential responses to changing watershed conditions. Importantly, the model is available
to inform future decision-making in the watershed.
The Watershed Analysis Risk Management Framework is a physically based approach to
loadings calculations which allows competent watershed analysis in spite of the lack of
actual loadings data. This means that the model can calculate the contaminant
contributions (e.g., nutrients, pollutants) to water bodies based on the physical processes
that occur between soil chemistry, pollutant runoff, hydrology, and other watershed
characteristics. In addition, WARMF can accommodate diversions in stream segments
such as with PG&E operations, and it can model septic systems within catchments.
WARMF also has a built-in decision support system with the ability to geographically
display data. This ability to display highly technical data in an easy to understand format is
important in a stakeholder-involved process such as this one. Finally, WARMF is a standalone GIS-based tool that does not require the user to have a license for ESRI’s ArcView
software.
Technical Aspects of WARMF
WARMF uses physically-based algorithms in a dynamic watershed simulation model to
calculate stream flow and water quality conditions within a watershed. In WARMF, a
watershed is divided into a network of land catchments (often called sub-basins), river
segments, and reservoir layers. Land catchments are further divided into land surface and
soil layers. The land surface is characterized by its land uses and cover, which may
include forested areas, agriculture lands, or urbanized areas. Daily precipitation, which
includes rain and snow, is deposited on the land catchments. WARMF performs daily
simulations of snow and soil hydrology to calculate surface runoff and groundwater
accretion to river segments. The water is then routed from one river segment to the next
downstream river segment until it reaches the terminus of the watershed. The associated
point and nonpoint loads are also routed through the system. Heat budget and mass
balance calculations are performed to calculate the temperature and concentrations of
various water quality constituents in each soil layer, river segment, and lake/reservoir
layer.
The first step in applying WARMF to the Upper Mokelumne River watershed was setting
up a basin map which defines land catchment, river segment, and lake/reservoir
boundaries. Basin map delineation for the Upper Mokelumne River watershed was
performed using the public domain tool BASINS 3.1 developed by USEPA. The digital
elevation model used in the delineation was provided by the project consultant team in
30m resolution. The stream network used in the delineation is the National Hydrography
Dataset downloaded automatically through BASINS. The 550-square mile watershed was
then delineated into 207 sub-catchments or sub-basins, 202 river segments, and seven
reservoirs/lakes. Given the potential for greater urban development in the South and
Middle Fork watersheds in future years, the lower part of the watershed was given a finer
resolution in sub-catchment definition than the upper regions.
A variety of input data were required to replicate Upper Mokelumne watershed conditions
in WARMF. The project Final Report (provided in the Documents section) discusses the
data development plan for the project and lists the data needed to utilize WARMF. Much of
the data required by WARMF was available from national and local databases and
included GIS data (e.g. land use / land cover) and non-GIS data (e.g., climate, air quality,
measured streamflow). Anthropological data were collected via interviews, field visits, and
additional research. Additional metals and nutrient monitoring data were also physically
collected on the South and Middle Forks of the Mokelumne River.
The WARMF model was then calibrated for the years 1999 through 2005 and validated for
1990 through 1999. The calibration and verification processes indicated that the model is a
fairly accurate representation of observed stream flow and water quality conditions. As an
additional calibration step, modifications were made to the land use inputs to account for
the Power Fire which occurred in fall of 2004. These results were compared with post-fire
monitoring data. These modifications enabled WARMF to successfully predict the increase
in parameter concentrations observed during the first wet season following the fire event.
Quantitatively, the stream flow and water quality calibration statistics vary from catchment
to catchment. The relative error of the stream flow simulation at Highway 49 was found to
be 4 percent, well below the targeted relative error of 10% for stream flow calibration. For
water quality, relative error varies both by catchment and by parameter, with a target
calibration relative error of less than 25%. Relative error for pH, temperature, dissolved
oxygen, total organic carbon, ammonia, and nitrate were approximately 0.1%, 5.9%, 6.0%, -1.0%, 15.2%, and 24.3%, respectively. It should be noted that models are only
theoretical tools used to represent a watershed as closely as possible to real conditions.
The decision support tools of WARMF can be used in future efforts to generate alternative
scenarios and graphically display cumulative water quality impacts of future land uses and
activities. WARMF is a stand-alone GIS-based tool that has been applied to over 20
watersheds in the United States and internationally. Additional background information on
WARMF is provided in the Final Report. The WARMF Technical Documentation and
User’s Guide are included with the WARMF application CD and are available from
UMRWA. The model must be used with the understanding that it replicates actual
conditions; it does not represent actual conditions.