Agricultural Yield Test

 

 

Yield Testing for Agricultural Purposes:

 

South Africa has very stringent regulations in place for the use of groundwater resources commercially.

Depending on your required daily extraction rate and the required application a Water Usage License (WULA) is required.

The process to follow:

  1. Siting and drilling of a new borehole

During this process detailed drill logs, water strike depths and borehole specifications is collected. Should the site have an existing borehole, that boreholes specific data needs to be captures ex. Depth, Static Water Level, Casing Depth, Coordinates etc.

  1. Yield Testing of the new or existing borehole

Depending on the use and extraction rate there are different parameters set out for the WULA application. However, the type of yield test does not vary.

There are five main steps to doing a proper borehole yield test for WULA applications.

a. Step Drawdown Test

The process wherein the borehole water is extracted at various rates over timed intervals. Usually 1-hour intervals until full drawdown is achieved.

b. Step Drawdown Recovery Test

Once the step drawdown test is completed a detailed log of the water levels recovery to original state is required. This process is normally specified at 90% recovery or a similar duration as the step drawdown test.

c. Constant Drawdown Test

Once all the step test data is processed a qualified technician or geohydrologist will determine an acceptable constant drawdown yield that has to be maintained for the duration of the constant drawdown test. The duration of the Constant Drawdown test depends on the required accuracy of the sustainable yield estimation. These tests are normally 12, 24, 48, 72- or 96-hours long budget dependent.

d. Constant Drawdown Recovery Test

Once the constant drawdown test is completed a detailed log of the water levels recovery to original state is required. The Recovery test is normally to 90% of original water level or the same duration of the constant drawdown test.

e. Reporting

A detailed report of the borehole yield test is compiled to reflect the rates extracted over time and the movement and recovery of the water levels. Without this data being accurately captured the yield test is pointless.

  1. Water Quality Analysis

During the yield test a water sample will need to be taken, normally at 80% of test completion. Depending on the application the sample needs to be tested to ensure that it is safe for use.

 

  1. Water Usage License Application

A qualified and registered geohydrologist will take all the above-mentioned data captured and determine a safe sustainable yield. This data along with the application is completed by the geohydrologist and the application is the lodged with the relevant department or departments.

  1. Water level monitoring

During the application process and after approval the relevant parties will be required to monitor the water level movement and recovery of the borehole while in use. It is paramount that the correct infrastructure is put in place to ensure accurate data can be captured and to ensure that your rights to use the water based on the licensing agreement does not get revoked. The only way to ensure that this is possible is through proper testing and equipment specification.