SPIS Toolbox - Analyze Water Extraction

ANALYZE WATER EXTRACTION

Water extraction must be based on a corresponding water withdrawal license that also provides for information on the allowed annual or monthly quantities/quotas and on specific conditions or restrictions such as seasonal limits. This step deals with the collection of information on the existing or planned water extraction approach (gravity, manual-lifting or motor-pump) and the water pump. Furthermore, the actual water availability must be evaluated. Both aspects are essential in determining whether an existing or planned irrigation system can be operated in a sustainable way.

The SAFEGUARD WATER 02 – Water Resource Management Checklisttool of this manual provides an orientation for the information and data to be collected and reviewed in Section 2. The analysis of water extraction potential from a well requires technical information that generally is provided by technical service providers (drilling contractor, pump manufacturers, irrigation system contractorsand pump installers).

The main aspect in this process is to evaluate the water availability in the selected water source. For surface water sources (pond/reservoir, lake and perennial river) a general assessment is undertaken as to whether the required water quantities can be provided in each month of the year. For wells and boreholes exploiting groundwater resources a test to estimate the hydraulic properties of the aquifer system is required in order to arrive at a sustainable abstraction rate. This is done by means of a pumping test (also called: aquifer test), which should be carried out after the installation of a pump. A pumping test is a field experiment in which a well is pumped at a controlled rate and water-level response (drawdown) is measured in one or more surrounding observation wells and optionally in the pumped well (control well) itself; response data from pumping tests are used to estimate the hydraulic properties of aquifers, evaluate well performance and identify aquifer boundaries. Typically, aquifer properties are estimated from a constant-rate pumping test by fitting mathematical models (type curves) to drawdown data through a procedure known as curve matchingand taking into consideration the geological set-up of the aquifer.

Based on the data obtained,key values related to the water abstraction (indicated as flow rates in m³/hour or m³/day) can be compared:

• Water withdrawal capacity: The quantities of water that can technically be abstracted from a water source with the installed abstraction/pumping device;
• Water withdrawal license: Maximum quantity of water a permit holder is legally entitled to in a given period of time (per year, month or day);
• Expected water withdrawal: The quantity of water expected to be needed according to the pre-planning of the irrigation system.

The objective of this exercise is to make sure that the pumping capacity:

• does not exceedcapacity indicated in the water permit/license;
• meetsthe crop water demand plus technical storage requirements.
  
  

Recommendation: Require your well contractor and pump installer to perform the analysis and provide the corresponding data before any further planning for the SPIS is undertaken.

Important: If one or more of the above principles with regard to the different flow rates are not applicable in the data comparison, the system cannot be operated on a sustainable basis and its operation may result in severe negative ecological (dried out well, negative water balance in the aquifer, dropping water table) and financial impacts (overdimensioning of system, insufficient water availability for agricultural production). A need to introduce adaptations to the system design or even abandon the project is thus apparent.

OUTCOME / PRODUCT

• Compilation of data and information on flow rates for water source, pump and system;
• Comparison of flow rates for safe yield of water source, water withdrawal license, pump and irrigation system.

Data Requirements

• Water source flow rate;
• Water license quota;
• Water pump flow rate curve;
• Expected water demand of the irrigation system.

Important Issues

• Safe yield (sustainable withdrawal) of the water source is the determining factor for a sustainable operation;
• Pumping or aquifer test must be performed and requires special expertise;
• Information on existing water sources and pumping installation is available with technical service providers.

People / Stakeholder

• Farmer and Agricultural Advisor;
• Water Resource Management and Licensing Authorities;
• Farmer Organization / Water User Group;
• Technical Service Providers.