Measurements of pressure head (h) and soil-water content (theta) were obtained at 8 depths at a study site located in a subhumid region over a period of 4 weeks. The site is vegetated with pasture grass and the soil is a silt loam. The collected data are contained in the spreadsheet (profiles.xls).

A zero-flux plane (ZFP) exists in the subsurface at a depth where dH/dz = 0, where H is total head, and z is the vertical coordinate. For analysis of 1-dimensional unsaturated-zone data, it is convenient to let z = 0 at land surface and increase downward. z is then equivalent to depth. Total head accounts for pressure head and depth and may be written as:

H = h - z

A) Determine the depth of the zero-flux plane (ZFP) for each measurement date. Select a depth interval that remains below the ZFP for all measurement dates and apply the ZFP method to estimate drainage (i.e., decreases in storage) from that interval between measurement dates. What is the total amount of estimated drainage for the 4-week period? Is the estimated amount of drainage different if data were available only for days 0 and 28? Suppose the pressure-head measurements have an uncertainty of +/- 0.02 m. How does this affect the uncertainty in the location of the ZFP and the estimate of drainage?

B) Daily precipitation data are available for the study site. Only two days had measureable precipitation: day 8 (60 mm) and day 12 (44 mm); no runoff was observed. Write a water-budget equation for the soil column. Calculate and plot cumulative estimates of water-budget components by using average daily values for each component.

Problem 5.2

A) With the data set described in Problem 5.1 (profiles.xls), apply the Darcy method to estimate drainage at depths of 2 and 3 m for each measurement date. The water-retention and hydraulic conductivity curves are described by the van Genuchten equations (Equations 5.2 and 5.3) with the following parameters:

ThetaS = 0.45

ThetaR = 0.067

alpha = 2.0/m

n = 1.41

Ks = 0.11 m/d

*** Note - when applying Equations 5.2 and 5.3, the values of pressure head (h) must be positive. The absolute value of pressure heads in the spreadsheet should be used in these equations. See Corrections for the text book***

B) Apply the unit hydraulic gradient assumption to estimate drainage at 2 and 3 m.

C) Compare and discuss the results from Problem 5.1 A and B and Problem 5.2 A and B.

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