Question

Problem 7.2.a) The Philip and Green-Ampt equations provide models for “infiltration capacity” (or “potential infiltration”). What assumptions are used in the development of the Philip and Green-Ampt infiltration capacity models? Clearly explain the difference between actual infiltration rate and infiltration capacity (potential infiltration). Under what specific conditions are the two the same?

b) State the condition that for a given rainfall (or snowmelt) event would never generate infiltration excess runoff. This should be written in terms of the rainfall/melt rate and the saturated hydraulic conductivity of the soil.Suppose the volumetric soil moisture at a particular site is measured to be 0.024. Immediately after the soil moisture measurement is taken, the region experiences a snowmelt event lasting 12 hours with a uniform melt flux intensity of 0.69 mm/hr. Assume that this region is uniformly covered by sandy loam soil. Similar to a rainfall event, a snowmelt event can result in infiltration and/or infiltration excess runoff. In the case of a snowmelt event, the melt flux/rate would be analogous to the rainfall rate during a storm event.

c) Based on the soil conditions/characteristics in this region, what values of the melt flux intensity would generate infiltration excess runoff? Based on your answer, will infiltration excess runoff occur during this snowmelt event? What is the physical meaning of the ponding time in infiltration? Will ponding occur during this snowmelt event? What will the cumulative infiltration be in this case? Justify your answers.

d) Assume the soil instead corresponded to one with the following soil hydraulic properties: saturated hydraulic conductivity of 0.00125 cm hr-1, saturated matric head of -21.8 cm, porosity of 0.435, and Brooks-Corey parameter b of 4.9. Is it possible that infiltration excess runoff will occur in the watershed given the same snowmelt event described above? Justify your response. Using both the Philip and Green-Ampt models, compute the time to ponding under these conditions. Will ponding occur for these conditions? Explain your reasoning.

e) Use the Philip and Green-Ampt models to compute the cumulative infiltration and infiltration excess runoff for this snowmelt event for the above mentioned initial conditions and soil properties from part d). Compute the cumulative infiltration (in mm) and infiltration excess runoff (in mm) with both models. Note: You should do so using the time compression approximation. What fraction of the total cumulative melt infiltrates vs. generates runoff for this case. By how much do the two models differ in their predictions?

Answer 1

Let's start with the answering questions one by one

Assumption made in the Philip and Green-Ampt model for infiltration capacity.

1. The Philip and GREEN Ampt models for the calculation of infiltration capacity was a semi empirical and semi thworatithe approach.

2. The is assumed to be two layered .

3 The texture of soil is assumed to be fine grained and coarse interlayer .

4. The infiltration is difeediff for the different burried depths and the hydraulic conductivity changes accordingly.

5. The soil profile must be of homogenous and this doesn't occur in the practical.

6.the infiltration rate decreases to a constant value when the wetting front crossed the upper infilteinfil which is remained unchanged after the wetting front advanced into the caorse layered.

Now the difference between Infiltration rate and Infiltration capacity.

Infiltration rate.- It is defined as the rate at which the water actually passes through the upper front of the soil to deep burried depths .it is expressed in m^3/second.

Infiltration capacity - it is the capacity of soil mass to store water with in their pores .it is also defined as the maximum amount of water that a soil mass allows to pass through it self