Soil water lab preparation

Associated lecture materials

To prepare for lab activities, first be sure you have reviewed the associated materials from lectures on soil water.

Contents of this module

This laboratory exercise includes a field data acquisition activity and a data analysis activity. The field activity is to get some hands on experience with the different methods for measuring the infiltration capacity of soils with different textures. The data analysis activity is to learn how to use spreadsheets for solving the Green-Ampt equation and processing the infiltration data.

Field data acquisition activity

The following file is the worksheet used for this lab (go to link and click download to get a copy of the PDF file). Please be familiar with the basic sequence of measurements we will be taking for each soil type.

Click this link to download the PDF file

Soils field worksheet.pdf

One of methods we will be practicing is the Philip Dunne permeameter test to provide an estimate of saturated hydraulic conductivity. This is just one example of many techniques in subsurface hydrology tied to inferring the hydraulic properties of porous materials by timing the change in head or volume in a column of water somehow hydrologically connected to the subsurface system (11:17 min).

The following is a commentated video of a Philip Dunne permeameter test to get you familiar with the nuances of successful completion of this technique. The diameter of the tube is 3.2 cm, if you would like to use data from this test to practice with the spreadsheets. If using the Munoz-Carpena spreadsheet, assume the change in water content is 0.3. As we will discuss, this number is not easy to measure, so you can try other values to see how uncertainty in this value would affect your estimate of saturated hydraulic conductivity (9:20 min).

Another device we will be using is a tension infiltrometer. Data from a tension infiltrometer can be used to estimate the unsaturated hydraulic conductivity relative to the ambient soil water content at the beginning of the test. These devices also provide an excellent demonstration of why gravity is actually not the force that is most important to estimating infiltration rates into unsaturated soils (12:29 min).

The manual for the mini disk tension infiltrometer provides more detail on the proper use of the device and instructions on how to use the spreadsheet for estimating the unsaturated hydraulic conductivity.

minidisc_manual.pdf

The following is the spreadsheet for processing the data from a mini disk tension infiltrometer experiment.

Click to download the Excel spreadsheet

minidisc_spreadsheet.xlsx

Data analysis activity

The first computer lab activity is to get familiar with a spreadsheet that solves the Green Ampt equation, and do a sensitivity analysis to understand how saturated hydraulic conductivity, antecedent wetness conditions, and rainfall intensity influence the evolution of infiltration rates though a precipitation event. You might want to review the following lecture materials about the nature of Green Ampt infiltration curves before getting into the details of calculations using a Green Ampt spreadsheet (7:47 min).

The Green Ampt equation is implicit in that the the integrated total amount of infiltration on the right side of the equation depends on the current infiltration rate your are trying to calculate on the left side of the equation. Therefore, the solution of the equation requires an iterative solver. We will use the "solver" in Excel to provide that iterative algorithm. You should download the following spreadsheet if you want to follow along with the video.

Click to download the Excel spreadsheet

Green-Ampt Calculator Ks_template.xlsx

And here is the video providing instruction on building and using the Green Ampt spreadsheet (26:28 min).

The second part of the computer lab is to process the data from the Philip Dunne permeameter tests using two different methods. You should download the following spreadsheet if you want to follow along with the video.

Click to download the Excel spreadsheet

Philip-Dunne Calculator Template.xlsx

And here is a video with instructions on how to use the Philip Dunne spreadsheet (11:31 min).