Irrigation ABSTRACT INTRODUCTION .I,

Transcription

1 .I, Irrigation LAND GRADING FOR SURFACE IRRIGATION OF SUGARCANE T. Chinloy The Sugar Manufacturers' Association (of Jamaica) Ltd. Jamaica, W.I. and J. A. Kelly The Nigerian Sugar Co., Ltd. Nigeria ABSTRACT The paper describes an experiment to evaluate the merits of precision land forming for irrigation under the conditions of 2 slopes, 3 furrow stream sizes and 3 furrow lengths. The experiment was carried out on 33 acres of a poorly drained, stru~tureless, heavy clay soil. Measurements were made of the water entering the experimental area, the rainfall and the evaporation from a U.S. Class A pan, and irrigation applied when 75% of the water available in the top 2 ft of soil was used. The experiment was continued over 28% months, during which a 16-month-old plant crop and 121/2-month 1st ratoon crop were harvested and yields recorded for each sub-plot. Better water use was obtained on the flatter (0.05%) slope than on the steeper (0.22%) slope. Furrow lengths of between 350 and 1050 feet were tested and the longer runs proved more productive than the shorter runs. There was no difference between using 70, 120 and 170 US gal/min, and it is likely that a furrow stream size of less than 70 US gal/min would produce as good results as the faster flows used. Using the concept that an acre inch of irrigation andlor useful rainfall should produce a ton of cane, high irrigation efficiencies, between 70 and 80%, were obtained. It appeared that precision land grading was the most important single contributory factor, as all combinations of slope, furrow length and furrow stream size tested gave relatively high irrigation efficiencies. INTRODUCTION The method of applying water by surface irrigation in Jamaica was described by Chinloy et al. (3). The method-furrow irrigation-allows for nonuniformity of the soil surface, as channels are placed ft apart at right angles to the furrow direction to carry irrigation water. The channels receive water from canals running parallel to the furrows, the canals being fed by a head main at the top of the field. The system has some disadvantages, the chief of which are impedance to equipment from the cross channels, difficulty of supervision and losses due to run-off. A preliminary examination (3) showed that longer lengths of run were feasible on heavy clay soils. The data indicated that the longer furrow lengths would not necessarily reduce yield, and they were easier to supervise and required less water for adequate wetting. It was found, however, that the grade

2 T. CHINLOY, J. A. KELLY 887 I down the furrows must be fairly regular to allow time for optimum and uniform wetting along the line, otherwise localized over-watering or inadequate wetting would result. Booher (1) stated that the application of water by surface irrigation methods requires that the land be smoothed so the farmer can control the water as it flows across his fields. He went further to explain the mechanics of the operation. In order to put the land surface to a smooth uniform slope, precision land grading is necessary. As this is an expensive operation, the purpose of this study was to evaluate the merits of the operation carefully. Accordingly, in the autumn of 1966 an experiment was laid out on 2 fields of an irrigated estate, with areas of 14 and 19 acres. MATERIALS AND METHODS The natural slope of the area in which the 2 fields are located is 0.25y0. The soil is classified as Rhymesbury clay (4), which is described as a poorly drained, saline, structureless, heavy soil with moderate to poor moisture supplying capacity and a relatively high water table. The important factors to test were furrow slopes, stream sizes and furrow lengths. The experiment was laid out with the following treatments: Furrow slopes: 0.05% and 0.22% Stream sizes: 70, 120 and 170 US gal/min Furrow lengths: 350, 700 and 1050 ft. The fields were surveyed on a 50-ft grid and calculations made of planes of best fit to give a furrow slope of 0.05y0 on the 19-acre field and of 0.22y0 on the 14-acre field. The work was carried out by bulldozers and a land plane (a scraper was not available). Each of the 2 fields was divided into 6 main plots to give 2 replicates of the 3 furrow stream sizes. The main plots were split to accommodate the 3 furrow lengths. The area was planted in September 1966 to var B 51410, a variety known to perform moderately well in the area and to be erect at yields of over 60 tonslacre. Planting was done on top of I-ft high banks spaced 5 ft apart. Germination was good. Irrigation was carried out on a water budget basis, water being applied when the fields were calculated to have used 2 of the available water in the top 2 ft of soil. The calculations were done on daily estimates of evapotranspiration (using an American Phn Evaporimeter), water holding capacity of the soil, rainfall, irrigation and an assumed 2-ft rooting depth. The plant cane crop, grown for 16 months, was harvested in January The 1st ratoon crop was harvested after about 12+ months growing period in February During these 2 cropping periods the island and the experimental area experienced one of the worst droughts in recorded history. A rain gauge is located about 1200 ft from the trial site, and the figures given in Table 1 tell the story. The annual average for was only in., but even this was considerably higher than the 1967 rainfall, and is kol-e than double that of the 1968 precipitation. To the 1967 rainfall must be added the 18 in. of rain which fell between September and December 1966 on the plant cane crop, making a total of 47 in. of rainfall for the 16-month plant crop.

3 Table 1. Monthly rainfall in inches January 1967 to December 1968 and average for previous3 years at Cooks Gate. Year Jan Feb Mar April May June July Aug Sept Oct Nov Dec Total

4 T. CHINLOY, J. A. KELLY 889 RESULTS RLUVL.IU I~rigation Usage Careful records were kept of the quantities of water used in irrigating 2, the trial (Table 2). When these figures are considered with the rainfall that Table 2. Irrigation data for plant and ratoon crops. Plant crop 1st ratoon crop 0.22 yo 0.05% 0.227, 0.05% I tem slope slope slope slope Total water metered into field (acre in./acre) No. irrigations applied Meail quantity applied/irrigation (acre in./acre) Mean requirement (acre in./hr/acre) Maximum requirement (acre in./hr/acre) Mean use of max. requirement (yo) fell on the 2 crops, over 90 in. of rainfall was used to grow the 16-month-old plant cane crop and just under 60 in. for the 12-month first ratoon. This is the water actually measured in the field plus the rainfall, and does not include water lost in conveyance from the source to the field. The first irrigation required considerably more water than subsequent ones. In the 14-acre field, 7.17 acre incheslacre were required and 7.63 acre inches were required on the flatter 19-acre field for the first irrigation as compared with an average of 4.8 and 6.1, respectively, for the whole plant crop. There appeared to be no surface drainage or internal drainage difficulties despite the removal of all infield drains; the uniformity of wetting was exceptionally good and the irrigators, who are accustomed to wetting a conventional twig and main system, were able to irrigate at a greater speed than normal and accepted the system readily. They were able to irrigate 0.37 acre/hr with a mean canal stream size of 1.3 cy ft/second. Study was made of the water use of varying furrow lengths, furrow slope and stream size; the data are summarised in Table 3. There is a suggestion that more water is used at the longest furrow runs when the highest stream size is applied in the plantfcane crop. This is not confirmed by the other figures. In fact, the amount of water applied for the-different furrow runs was roughly the same for the 2 crops, varying by a total of 3 acre in./acre for the plant cane crop and 2 acre in./acre for the ratoon crop. The quantity of water applied varied with the stream size, the greater the stream size the greater the amount of water applied. In the plant crop the stream size of 70 gal/min used an average of about 3 acre in./acre less than a stream size of 170 gallmin; the comparable figure for the 1st ratoon crop was 5

5 890 IRRIGATION Table 3. Gross in. of ilrigation water measured at entralice to experimental area for varying flirrow lengths, furrow slopes and stream sizes. I Plant crop 1st ratoon crop Length of run (it), Length of run (ft) Stream size Furrow (US gal/rnin) slope (%) i05d acre in./acre. The greatest difference was observdd in the furrow slopes. The quantity of water applied on the steeper slope was bktween 70-75% of that applied on the flatter slope in both plant and 1st ratoon. Cane Yields There was little difference in the quality of cane for tlie different treatments. Results are presented in Tab16 4 for the tons canelacre only. The dif- Table 4. Tons of canelacre for stream size, furrow slope and furrow length for plant and 1st ratoon crops. Plant crop 1st ratoon crop -- Length of run (ft) Length of run ( 1) Stream size Furrow (US gal/rnill) slope (%) 350 7' I G0.6 6j G Standard error percent general mean: Plant 1st ratoon Slope Furrow run Stream size

6 T. CHINLOY, J. A. KELLY 891 ference in yields between the 2 slopes is significant in both crops, the flatter field giving an average 4.5 and 4.8 tons of canelacre more than the steeper field in the plant and 1st ratoon crop, respectively. The interaction between slope and furrow length is interesting; the data are summarised in Table 5. Somewhat con- Table 5. Average tons of canelacre for plant and first ratoon crops for slopes and furrow lengths. Length of run (ft) trary to expectations the longer runs produced more cane than the shorter runs; this effect was more noticeable on the steeper field where the difference in tons cane per acre between the shortest and longest runs was 7.l/crop, as against 3.2 for the flatter dope. Furrow stream sizes tested in the trial are from 3-7 times the average flow normally used. The influence of slope and length of run on the yields from different stream sizes is presented in Table 6. There was little overall difference 7 able 6. Tons of canelacre for stream size and slope, and stream size and furrow length averaqed over plant and first ratoon crops. Slope (%) Length of run (ft) Stream size (US gallacre in.) in yields from plots receiving water at different furrow stream flows, though there was a trend in favour of the lowest furrow stream size. At the longer runs, similar yields were obtained froin the 3 furrow stream flows, but at the shortest run (350 ft) 70 US gal/min/furrow gave higher yields than the 2 bigger stream sizes. Chinloy et al. (3) drew attention to the drop in yieid in the middle of the line compared dith the yield at the top or bottom of the furrow when long line irrigation is practised. Their maximum stream size was 33.3 gal/min and the furrow run was 660 ft. Yields were measured at different points along the line for each treatment, In nearly every case a similar effect was obtained irrespective of furrow slope, furrow length and stream size. Taking an average over all these 3 factors, the third oi the area upstream yielded 51.0 tons of canelacre, the middle third 45.8, and the bottom third 50.7.

7 892 IRRIGATION Irrigation Efficiency Cleasby and Hill (2) and workers in Hawaii have concluded that about 1 ton of cane is produced by the evapotranspiration of 1 acre in. of water. Therefore, irrigation efficiency may be measured by the comparison of cane yield to gross inches of irrigation plus useful rain. Rainfall minus run-off and deep percolation gives a reasonable estimate of useful rainfall. If rainfall is not considered, high average returns of 1.34 ancl 1.23 tons of canelacre inch irrigation water applied were obtained for plant and first ratoon, respectively. This is important in giving a guide as to what might be achieved in an area where water is becoming progressively limiting. The data are all the more important as they were obtained during 2 particularly dry years. Irrigation efficiences, determined by considering gross irrigation and useful.rainfall, are given in Table 7. Table 7. Irrigation efficiencies for plant and 1st ratoon crops according to furrow slope, furrow run and stream size. Plant crop 1st ratoon crop Length of run (it) Length of run (ft) Slream size Furrow (US gal/min) slope (%) Using the same method and making a generous allowance for non-use of pumping capacity and conveyance losses, commercial surface irrigation attains an efficiency of about 42% on Jamaican sugar estates. Far higher efficiencies were obtained in the experiment of which most of the increase must be attributed to precision land grading, though better supervision and higher stream sizes must have played a part. It is also evident that there was no loss of efficiency up to the longest (1050 ft) run. DISCUSSION The conventional system of surface irrigation practised in sugar agriculture in Jamaica gives efficiencies of about 40y0. It had been demonstrated (3) that longer lines were feasible on heavy clay soils provided the land was uniformly graded, and it was felt that regrading of many existing fields might be necessary for the efficient use of long line irrigation. The experiment confirms that uniformity of grading, at least between slopes of 0.05 and 0.22%, will improve the efficiency of water use appreciably. In Jamaica there is a surface irrigated belt which accounts for over 40%

8 T. CHINLOY, J. A. KELLY 893 of the sugar produced in the island. Water is becoming as limiting to sugar production as good land in that area, and already the recurrent cost of irrigation is higher than the rental value of land. It is, therefore, very important that high production/unit water is obtained. Precision land forming offers a means of effecting this, though it is appreciated that there are large areas which may not be capable of being land-levelled economically because of topography, and other areas where the shallowness of the top soil would preclude its use. We mentioned that irrigators accepted the system readily and were able to irrigate at a faster rate, perhaps of the order of 5 times as fast, than with the conventional system. In addition the system permits the elimination of all infield drains, thus reducing the cost of drainage. Because of the elimination of drains and canals, tractor maintenance is considerably reduced, the productivity of the land increased if only by reason of having the area in drains put into cane and all mechanical operations such as inter-row cultivation, herbicide application and fertilizer application facilitated. Further, the land is put in its best form to accommodate mechanical harvesters. The yields obtained were compared with those obtained in the past and, judged on a productivity of canelunit time, were appreciably better. It would appear that the worst treatment was superior to common practice. It therefore seems desirable that levelling be started on fields where permissible slopes are between 0.05 and 0.25y0 and the furrows run the length of the fields. ACKNOWLEDGEMENTS The authors thank the management and field staff of Sevens Limited for their wholehearted co-operation in this venture. Thanks are also due to the field staff of the Sugar Research Department for their efforts in harvesting the trial, and particularly to T. Peart who spent many long hours supervising the irrigation. REFERENCES 1. Booher, L. J Land grading. World Farming 12 (11) : Cleasby, T. G., and J. N. S. Hill The overhead irrigation of sugarcane in Natal. Proc. ISSCT, Chinloy, T., T. C. E. Wells, N. J. Chin, and J. L. Ramsay Possibility of long-furrow irrigation under heavy clay soil conditions. Proc. ISSCT, 11: Soil and Land Use Surveys. No. 7. Tamaica. Parish of Clarendon. The Resional Research Centre of the ~ritish ~aribbea; p. 14, 27 and 32.