Water Harvesting

New Zealand Journal of Agriculture, Volume 95, Issue 3, 16 September 1957, Page 223

Water Harvesting

By

P. B. LYNCH,

Crop Experimentalist, Department of Agriculture, Wellington

AT the McGarvie Smith Animal Husbandry Farm (University of Sydney), the officer, In charge, Mr. H. J. Geddes, has developed a project which he terms "water harvesting". This is a method that could have wide application both in Australia and New Zealand in many districts where the seasonal distribution of rainfall is erratic and where droughts are a major factor limiting production. This article is based on data supplied by Mr. Geddes during an inspection of the farm by the author.

THE animal husbandry farm at Badgery Creek is some' 25 miles inland from Sydney and has an average yearly rainfall of 25in. However, the seasonal and yearly variation of this rainfall is very great. In 1954, for instance, there was a drought of 8. months. Normally two heavy falls of rain occur each year, but they may come at any time. Such erratic distribution of rainfall has meant that any temporary success with pasture improvement has been lost during dry spells, when the sward reverts to poor native species. The soil type is a rather infertile heavy clay, derived from shale, which responds to phosphate and molybdenum. However, this type of soil, being rather impermeable to water, is ideal for the construction of waterholding dams.

It was clear in the early stages of this project that the key to improvement of the country was irrigation, but the difficulty was the supply of water. Bore water was far below the surface and was brackish. Two creeks on the property ran for only a few days during storms and then carried so much water that costly concrete weirs would be required to dam them. Under these circumstances Mr. Geddes conceived the idea of using run-off water for • irrigation: The first dam constructed had a capacity of 6 million gallons. It cost £A2SO, or £AII ss. per acre-foot. Other dams have cost slightly more, but much depends on the natural contour of the

country. Dams are constructed., in natural catchments and are fed by surface run-off water from such catch? ments and by water collected from adjacent slopes by contour ditches/ A series of.. dams is constructed at suitable sites down these catchments so that , the overflow from each is collected by lower dams. Water from a creek that runs about twice a year has been saved by z socalled “turkey nest tanks”. These are built on flat ground near the creek and are filled by water pumped from the creek when it is in flood. Each ' tank consists of . a circular earth wall built above ground .by spoil obtained from immediately within the tank. The centre is not disturbed. .. The diagram above shows the method of construction. . . ' ... . A clay subsoil is necessary for these tanks. The surface soil is removed both from the base of the bank and from the area used to form the bank. Clay is built on clay. The surface soil can be built to “face” the outside of the bank after construction. . Shallow tanks store water cheaper than deep ones, but a certain minimum depth is needed. Mr. Geddes considers the minimum capacity to be 5 million gallons. The larger these tanks . are the cheaper is the cost of water storage. One of the dams at • Badgery Creek holds 8J million gallons of water for an excavation of only 8000 cubic, yards.

Water is stored at a capital cost of about £A2I an acre-foot. Cost of filling the dam is Is. worth of diesel fuel per acre-foot for every foot in height the water is lifted. Necessary Conditions for Water Harvesting Two essential requirements for water harvesting are (1) good waterholding soils and (2) suitable topography. Heavy clay subsoil is a great advantage in eliminating trouble from seepage and tunnelling, and gently undulating country is ideal in that a large storage of water is possible for each cubic yard of excavation. However, there is no doubt that sites for dams can be found under a variety of different conditions, though flat land is necessary for the turkey nest tank. Evaporation from the dams has been much less serious than was at first thought. As the shallowest parts of the dams are used up first while irrigating, this reduces the water surface for evaporation. The storage system at Badgery Creek holds 48 million gallons of water, and it is considered that this would last through a 12 months’ drought. Practically the entire farm of 400 acres is now drained into these dams, and about 200 acres are under irrigation. It would be safe to allow a storage of 1 million gallons for every 3 acres irrigated. In most of New Zealand, however, where droughts are less severe than at Badgery Creek, it is probable that 1 million gallons would irrigate a considerably greater area. Irrigated Pastures Irrigated pastures seen were dominantly white clover with some red

clover, subterranean clover, and strawberry clover in the wetter parts. Growth was extremely vigorous. The grass (mostly short-rotation ryegrass) was rather sparse, but all pastures seen were young, and the oldest of these (3 years) was getting a better balance of grass to clover. Pastures were strip grazed with use of an electric fence. On the 200 acres irrigated some 150 head of dairy stock were being carried, 60 of these being milked at the one time. The high proportion of young and dry stock is due to the fact that the herd size is being substantially increased. The farm supplied whole milk to Sydney, and a few cows were calved-down each month. About 70 sheep were carried in addition. Pasture mixtures sown were generally 131 b. per acre, comprising 61b. of short-rotation ryegrass, 2^lb. of white clover, 21b. each of red clover and subterranean clover, and jib. of strawberry clover. Cocksfoot, paspalum, and timothy were under trial. Manurial treatment was 4cwt. of superphosphate per acre in the first year in two dressings of 2cwt. each, plus 2cwt. a year thereafter. Molybdenised superphosphate was used when sowing down to pasture and occasionally afterward, as the soil is somewhat responsive to molybdenum. Methods of Irrigation Sprinkler systems were used for irrigation, the water being pumped out of the storage dams nearest to the paddock being irrigated. The farm has 45 chains of spray lines and, with 60ft. moves, three men were kept busy irrigating much of the year. At pre-

sent the usual rate of watering is lin. every week, but it is intended to in-

vestigate various times, frequencies, and rates of irrigation. It is considered that on a one-man farm 10 to 20 acres could be irrigated, and the labour requirements for this, though considerable, should be set against the fact that smaller amounts of conserved feed should be required. Application in New Zealand “Water harvesting” would seem worth while in much of New Zealand, particularly those districts with a rainfall of 30in. and under or where summer drought is frequent and serious. It offers a means of water conservation and reducing run-off which will interest all concerned with floodcontrol problems. Water for irrigation is obtained cheaply and costs of application are relatively low, as irrigated paddocks can be located close to the source of supply. If the paddocks are at a lower level than the water in the dams (as can frequently be arranged), pumping costs are lowered. The huge capital costs of most irrigation schemes are eliminated and water may be made available in districts which such schemes cannot hope to service. Much of the foothill and downland country of Canterbury and Otago in particular would seem to be well suited to water harvesting. However, many problems remain to be solved before the method can be properly evaluated. Farmers would be wise to study the project carefully, and if their conditions appear suitable, to begin in a small way first, as no doubt modifications would be necessary in different districts and on different soils.