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SEPTEMBER 1996 INFILTRATION PROJECT PROPOSAL
PURPOSE: To determine infiltration rates of several selected sites within
the watershed of the Northwest and Southwest Valleys
PERSONNEL: Robert M. Dixon, Ph.D and Ann B. Carr
The Imprinting Foundation
PROJECT DATES: Initiation - 16 Sep 96
Completion - 21 Sep 96
BACKGROUND:
Infiltrometers, even the largest, measure point infiltration rather than non-point or watershed infiltration. Most infiltrometers yield infiltration rates that are unrealistically high relative to point infiltration rates under natural rainfall because of lateral flow of both air and water. Closed-top infiltrometers, however, induce counterflow of soil air during infiltration, thereby minimizing the effect of air lateral flow (Dixon, 1995 and references therein).
Watershed infiltration tends to increase with areal scale and the micro and macroroughness of topography. Obviously, the time during which infiltration can occur increases with the size of the watershed. Also, the surface area through which infiltration occurs increases with surface roughness. Not so apparent is the fact that surface roughness provides elevated areas for easy escape of displaced soil air.
The watershed size-infiltration relationship is especially profound in arid and semiarid mountainous regions where rainfall is often intense, of short duration and of limited areal extent. Such rain often occurs in the upper reaches of the watershed (orographic effect) producing flash floodwaters that subsequently infiltrate in downslope flatter parts of the watershed. Additionally, large quantities infiltrate enroute via fissured rock in the bottoms of water courses. Such infiltrating water quickly becomes groundwater by bypassing the storage space in the unsaturated zone above. Thus, in these watersheds point infiltration is relatively low compared with watershed infiltration when expressed as a percent of rainfall infiltrated. In fact on drier-than-normal years these watersheds lose very little, if any, rainwater by surface flow. Rapid bypass infiltration may also occur in water courses during snowmelt.
Point infiltration rates, as measured with infiltrometers can help evaluate the relative contribution of water courses and the land between the courses to groundwater recharge. Most recharge probably occurs in the water courses because of the prevalence of the set of conditions favoring bypass infiltration which is an abundant supply of water with sufficient hydraulic head to drive it rapidly down the underlying rock fissures into the groundwater. Displaced air is readily exhausted along the sides of the water courses which are relatively high and dry.
PROCEDURE:
• Select three key sites for making infiltration runs within the locations:
- constructed cover over the tailings impoundments
- natural surface between water courses
- natural surface in bottom of water course
• At each site make triplicated infiltration runs with closed-top and open-top infiltrometers operated side-by-side. Use 8-inch (20-cm) infiltrometer cylinders with a 4-inch (10-cm) wide buffer ring of sieved and saturated soil. For more details see Dixon, 1975 and references therein.
• Record infiltration at 5-minute intervals and continue infiltrometer runs for 60 minutes or 8 inches (20-cm) of total infiltration whichever occurs first.
• Analyze and interpret data by least square fitting to Kostiakov's equation or a power function of time in the form I = AT B where I is cumulative infiltration in inches (cm), A is the 60-minute infiltration amount and B is a variable, the size of which indicates the deceleration in infiltration rates (Dixon, et al. 1978 and references therein). Compare open and closed-top infiltrometer data to determine the importance of hydraulic head at the soil surface.
REFERENCES:
Dixon, R.M. 1975. Design and use of closed-top infiltrometers. Soil Science
Society of America Proceedings 39:755-763.
Dixon, R.M. 1995. Water infiltration control at the soil surface: Theory and
Practice. Journal of Soil and Water Conservation. 50:450-453.
Dixon, R.M., J.R. Simanton, and L.J. Lane. 1978. Simple time-power functions
for rainwater infiltration and runoff. Arizona-Nevada Acad. Sci. and
America Water Resources Association 8:79-89.
PROJECT COST:
The cost is estimated at $5,000 which is based on 5 days in the field to collect data. It includes all personnel, equipment, supplies and travel costs but does not include items No. 4 in the procedure (data analysis and interpretation). Infiltration equipment and supplies used in the project will remain the property of The Imprinting Foundation.
The Imprinting Foundation
1616 E. Lind Road
Tucson, Arizona 85719