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Reporting On The Maintenance Of A Water Quality Control System
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Water quality varies throughout the year. When reporting on the WQCS, it is important to, first of all, decide when a deviation in water quality is large enough to be alarming, and then to determine whether deviations are due to natural variation, or to contamination.
To do this, one needs a set of standards for irrigation water used on the specific crop and historical data to set standards for the particular water source. This means that a WQCS is required for every water source utilised, especially for those sources that are only being used when the usual or main source is depleted.
Detecting deviations should prompt actions to contain or address the deviations in order to minimise their effect on crop production.
Water Quality Standards For Crop Protection
Water quality standards are only of value to indicate limitations, because one seldom has a choice between water sources. Where more than one source is available, mixtures of good and moderate quality water must be used to increase the volume of acceptable quality water.
Water like this is however scarce and therefore care should be taken to obtain data for each specific crop’s sensitivity and requirements with regards to water quality. The following table is an extract from the Irrigation Design Manual and gives general guidelines for allowable levels of salts and minerals commonly found in water.
The Department of Water Affairs and Forestry developed the South African water quality directives to provide water quality information to water users. Volume 4 of the directives specifically deals with irrigation water and contains criteria for the evaluation of water by indicating the acceptable levels for specific elements and other parameters in four classes. The document is available from the Department.
A summary of the directives is included in the Table below.
Table: Various limits for specific problems with irrigation water
Historical Data For Received Water
Historical data of the composition of received water is the most important. This information determines the actions required to optimise water quality. Information on the effluent when rinsing the pipes, backwashing, contamination, and output water is also informative for managing the complete water system. Using this information, seasonal and long-term variations can be studied.
Follow the process below as a guideline to collect your data.
Example: Historical Water Data, long term average rainfall.
|
Date |
pH |
EC |
SAR |
Cl |
HCO3 |
|
Optimal |
<8.50 |
<125 |
<1.00 |
<2.00 |
<1.50 |
|
26/07/00 |
7.24 |
74 |
0.52 |
1.62 |
1.67 |
|
24/03/01 |
6.35 |
45 |
0.49 |
0.51 |
0.62 |
|
|
|||||
|
27/07/01 |
7.18 |
82 |
0.72 |
1.83 |
1.58 |
|
25/03/02 |
6.55 |
51 |
0.41 |
0.45 |
0.55 |
|
|
|||||
|
24/07/02* |
7.55 |
99 |
0.92 |
1.93 |
1.85 |
|
26/03/03 |
7.05 |
71 |
0.55 |
0.62 |
0.78 |
|
|
|||||
|
24/07/03 |
7.67 |
110 |
0.99 |
1.95 |
2.01 |
|
25/03/04 |
7.07 |
82 |
1.10 |
0.82 |
0.93 |
Rainfall has a marked influence on the quality of most water sources, and therefore the record should include some sort of reference to the rainfall for the ensuing period or season.
Also, note the two dates chosen for sampling. This farm is in the summer rainfall area. Sampling in July is done before the summer rain starts and will reflect the worse condition of water quality. The sampling in March is after the summer rains and should reflect the best quality available.
The major conclusions that can be drawn from the example data are the following:
- The quality of the water fluctuates significantly between summer and winter.
- The quality of the water source is deteriorating.