Soil moisture is modeled through the stream flow modeling process. In order to properly simulate or forecast stream flow, hydrologic models must also simulate soil moisture over the river basin of interest to determine the amount of surface runoff. These plots show simulated soil moisture derived from a spatially gridded hydrologic model used to simulate and forecast stream flow. The soil moisture values shown are subject to modeling constraints and uncertainties and may not accurately reflect measured soil moisture.

Southern Arizona -- Fractional Water Content
Click Map to View Grids

The Colorado Basin River Forecast Center is implementing the National Weather Service Hydrology Lab Research Distributed Hydrologic Model (HL-RDHM.) This model simulates basin response on a 16-km2 (4-km by 4-km) grid. In regions where the basin size is often larger than the spatial extent of the storm cell, distributed models can be a better method of simulating the hydrologic response because the intensity and extent of the rainfall observations are preserved. A by product of the HL-RDHM is the ability to produce grids of soil moisture states.

Observed stream flow is available from a wide network of river gages. The model is calibrated so that the simulation closely matches the observations and modified as necessary so that it matches the most current observations. By utilizing the Sacramento Heat Transfer Model (SAC-HT) algorithm, detailed grids can be produced. This algorithm was developed by Victor Koren* (Office of Hydrologic Development) to provide a model for frozen ground related hydrologic problems. The model yields a set of frozen and non-frozen soil moisture data. For the test region, temperatures suffient to produce freezing soils rarely, if ever, are observed. SAC-HT estimates fractional water contents at user specified depths below the surface. Fractional water content is the ratio of liquid water volume to the total soil sample volume. Values are assigned after the SAC-SMA module is run and are computed via weighted averaging from the computational layers. This output is plotted as either a basin average time-series, outlet time-series or displayed as spatially registered grids for each depth. It is expected to show periods of recharge to rooting zones and as a method to forecast short term moisture depletion when little precipitation is forecast. It is important to remember that these grids represent native soil moisture in regions that are not artificially wetted through irrigation methods.

The Upper Verde (Main stem Verde River and tributaries above Horseshoe Reservoir) and the Santa Cruz / San Pedro river basins were selected for their relatively, extensive precipitation and stream flow gage networks, complete weather radar coverage, and lack of major regulation. As calibration data sets are developed, we plan to extend this model to additional regions in the Upper and Lower Colorado River Basins.

* Koren, V.I., 2005. Parameterization of Frozen Ground Effects: Sensitivity to Soil Properties., Seventh IAHS Scientific Assembly. IAHS Publication Non 303., Foz do Iguacu Brazil, pp 125-133.

This is a preliminary effort to produce near surface soil mositure status from a hydrologic model. Work continues on both the model and display interface. Feedback is encouraged. Please feel free to contact Ed Clark