A Guide In Determining When To Irrigate

Press, Volume XCIX, Issue 29324, 1 October 1960, Page 8

A Guide In Determining When To Irrigate

The farmer using irrigation is faced with the problem of determining how much moisture there Is in the soil so that he can decide whether the soil has dried sufficiently to warrant another irrigation, and whether the fast rain waa heavy enough to postpone irrigation until the next series of watering days. Measurement of moisture in the soil seems the obvious answer, but, unfortunately, the methods which can be used to measure soil moisture (and there are dozens of them) are either very slow or not particularly reliable. There is, however, another approach. The rate at which soil dries out depends mainly on the weather, the most important aspect of which is the amount of energy received from the sun. Moisture lost from the soil can be estimated from information obtained from meteorological readings, and daily changes in the soil moisture undir a healthy pasture can be readily estimated. At the Winchmore irrigation research station the methods of doing this have been thoroughly examined and Mr D. S. Rickard, a senior scientific Officer, and Mr P. D. Fitzgerald, a scientific officer who is associated with him, say it has been found that if they had used a method based on weather data for experimental irrigations, in nine out of 10 cases they would’have been nd more than two days out either way; The method involves a daily estimate of evaporation from the pasture and the soil, and this, combined with rainfall figures, gives a day-by-day measure of total moisture lost from the soil. The method is described as being a great deal simpler than ordinary bookkeeping. At the foot of this article there is part of a sample form which can be kept for recording the moisture position in the soil It is assumed here that water is available once a week under the roster system. (If a farmer has an independent water supply, as with spray irrigation, thia metriclion does not apply->

The first column lists the average daily loss of moisture from the soil under a good pasture, and the second column is for entering daily rainfall figures. Starting*at the beginning of the season (September) or after irrigation, a day-by-day calculation is carried out by adding together the water lost and taking off the rainfall from the day-by-day total. The amount of water lost (in toe sample, 11 points per oay) is always added in, whether rain falls or not.

In the sample given, it was assumed in the first balance column that irrigation was applied , on November 30, and the calculation was started on December .1. The water lost on December 1 was-11 points and on the second another 11 points was lost, making a total loss of 22 points. On December 3, a further 11 points lost gave a total of 33 points, but 8 points of rain fell, making the actual balance oply 25 points. This simple arithmetic is carried on until it is considered that the balance (or deficit) has built up to a level where irrigation is required.

Where there is a deficit of less than 50 points irrigation is not considered necessary; at 50 points irrigation may be applied for maximum continuous growth; irrigatiotf when the deficit reaches 100 points represents-a high level of watering and for practical purposes highly efficient pasture production; with a deficit of 150 points irrigation is considered necessary to obtain adequate pasture response and to avoid checks in growth; at a deficit of 200 points irrigation is essential, thta representing a period when the pasture is suffering from

definite’ water shortage; and where the deficit is above 200 points considerable pasture production losses will already have occurred and two irrigations might be needed to restore pasture growth rate. In the second example given (note the second balance column), the month has begun with a deficit carried over from "the previous month. With water available on December 7, and a deficit of 89 points by that date, the farther decided to irrigate as he anticipated being busy at the time of the next rpster. He irrigated this area and started his calculations again. However rainfaH on December 10 was greater than the deficit which had built up by that date, and the calculation had to be begun again on December 11.

It should be stressed that this method is given as a guide to farmers as to when to irrigate, and is intended to assist them with this problem. It is only the farmer who is in a position to weigh up the pros and cons of irrigation against labour requirements for other work, feed requirements and stocking rates, etc. If this method gives the farmer an additional factor (soil moisture conditions) which will help him to make the correct decision, it will have fulfilled its objective. At present the method should only be used for pasture, and on well-drained soils. A farmer wishing to use this method would need a reliable rain gauge, and a supply of forms. Copies of forms for SeptemberApril, together with instructions, can be obtained from the Winchmore Irrigation Research Station. Private Bag, Ashburton. IRRIGATION REQUIREMENT SAMPLE •

December Date Water Water Balance Balance Lost Gained Pts. Pts. 1 11 __ • 11 41 2 11 __ 22 52 3 11 8 25 55 4 11 10 3 56 5 11 37 67 6 11 —— 48 78 7 11 __ 59 •89 8 11 —*> 70 11 9 . 11 —— 81 22 10 11 50 42 11 11 — 53 11 •Irrigation applied.