Introduction
Water ressources management is an important issue of our days, and especially in arid and semi-arid regions, where a better estimation of water balance in soil is crucial. One important issue is to evaluate qualitatively and quantitatively the spatial distribution of water in the region of interest. The input is generally the precipitation, but we need better tools to evaluate how water redistributes itself whithin the soil surface. Soil infiltration models are one way of dealing with this issue.
 

Objectives
 
 

• Evaluate infiltrability rate, using soil survey data the types of soils in Tooele County, and their physical characteristics;
• Delineating the areas were rainfall intensity overcomes the infiltration rate, therefore runoff will occur.
• Calculate (based on simple model) soil water storage in different areas, for a given rainfall event - focus on Rush Valley area.

In order to evaluate the infiltration rates for water infiltration in different types of soils, the Lewis-Kostiakov equation was used:

Data used and procedure

Soil data in digital format for the area was downloaded from the United States Geological Survey (USGS) web site. Data is organized in 7.5 quadrangles an can be added as map layer in ArcView. For the area of interest, 6 quadrangles were used (coded in USGS classification as S4011236, S4011237, S4011238_0a, S4011244_0a, S4011245_0a, and S4011246_0a).

Data characteristics are as follows:

        Grid_Coordinate_System_Name: Universal Transverse Mercator
        Universal_Transverse_Mercator:
          UTM_Zone_Number: 11
          Transverse_Mercator:
            Scale_Factor_at_Central_Meridian: 0.9996
            Longitude_of_Central_Meridian: -117.0
            Latitude_of_Projection_Origin: 0.0
            False_Easting: 500000
            False_Northing: 0.0
        Universal_Transverse_Mercator:
          UTM_Zone_Number: 12
          Transverse_Mercator:
            Scale_Factor_at_Central_Meridian: 0.9996
            Longitude_of_Central_Meridian: -111.0
            Latitude_of_Projection_Origin: 0.0
            False_Easting: 500000
            False_Northing: 0.0
      Planar_Coordinate_Information:
        Planar_Coordinate_Encoding_Method: coordinate pair
        Coordinate_Representation:
          Abscissa_Resolution: 0.61
          Ordinate_Resolution: 0.61
        Planar_Distance_Units: meters
    Geodetic_Model:
      Horizontal_Datum_Name: North American Datum of 1927
      Ellipsoid_Name: Clarke 1866
      Semi-major_Axis: 6378206.4
      Denominator_of_Flattening_Ratio: 294.98

The layers were merged into a single one, called RV.lyr.
In the attribute table, the last field is MUSYM, which is a symbol that uniquely identifies a map unit within a soil survey area. According to this last field classification of the area, a new field called INF (from infiltration) was added. This field contains values of infiltration rates calculated primarly in Excel for each type of soil, using the model described above. Based on this field, a new delineation could be done in ArcView, considering the areas where rainfall intensity (assumed constant) overcomes infiltration rate as regions with runoff.

The scenario includes the hypothesis of three different rainfall events:
1. Intensity: 1 mm/h; time: 2 h;
2. Intensity: 5 mm/h; time: 2h;
3. Intensity: 5 mm/h; time: 4h.

(Rainfall intensities are assumed constant in time.)

Calculations of Lewis-Kostiakov model parameters and of infiltration rates were done in Microsoft Excel:

 

Results and discussion

For the first case, a raifall event with intensity of 1mm/h and a duration of 2 hours, calculated potential infiltration rate is in between 40 to 75 mm/h (really high!) so all water from precipitation infiltrates in soil, no matter of the soil type. No runoff occurs in this situation.
The second simulation was done for a rainfall intensity of 5 mm/h, during a 2 hours period. In this case, calculated infiltrability ranges from 7.5 to 13 mm/h.

Regions in blue, are the regions where there is runoff: a) for 2 hours and b) for 4 hours.
One can see the expected extension of surfaces where runoff occurs. Next steps for future work could be to evaluate it quantitatively and integrate the total volume of water that accumulates from one region with runoff to its adjacent regions. This will be important for the global and local water balance in soil, as well for calculating soil water availability for plants. As more data is available, the model shoud be improved and take into account evapotranspiration, land coverage, slope, etc.
 

Conclusions
 

• Simple infiltration model can be used to delineate the regions where runoff occurs in different weather conditions scenarios.
• GIS tools are useful to evaluate water availability for plants in different climatic conditions, by simple computer simulations.
• The model can be improved by introducing more constraints in the water flow processes (lateral flow, plants uptake etc.)

References

- Or D., and Wraith, J.M.,  1994. Agricultural and Environmental Soil Physics (classnotes).
- Zeiler, M., 1999. Modeling Our World - ESRI Press.