Post evaluation of on-farm irrigation systems for the Glenmark community Irrigation Scheme : A thesis submitted in partial fulfilment of the requirements for the degree of Master of Applied Science at Lincoln University

Zimba, Jairos Limited Daniel
Fields of Research
ANZSRC::300201 Agricultural hydrology
Many community irrigation schemes built by the Government in New Zealand involved large investment costs. However, very few of them have been evaluated after construction. This does not imply that the schemes have always been successful; rather, some Government priorities have frequently changed and directed to new schemes instead. One such scheme that has not been evaluated is the Glenmark scheme in North Canterbury. The scheme was originally planned for 28 storage dams on 20 farms but only ten were finally constructed due mainly to increased costs. Of these (ten farms), only farms were studied to some detail (belonging to Messrs B. Moore, K. Stackhouse, J. Corbett & B. Harris). The scheme was the first of its kind in the country to be designed and built on "water harvesting" principles. As such, many problems encountered were peculiar to the scheme and had never before been experienced using traditional methods. The objectives of this study were to evaluate the procedures followed, propose alternatives, assess the on-farm irrigation activities, suggest improvements and review the social and environmental implications. The as-built scheme was compared with what was intended. Problems encountered with the new concepts were also examined. IRRICAD (version 5.3, 1993) and CROPWAT (version 5.7, 1991) computer programs were used to assess the engineering designs on four farms in the scheme and to assess the existing water management practices respectively. Chapters 1 to 3 show the need for irrigation in the Canterbury region and a requirement for the Government to administer irrigation development. The benefits of irrigation to both the farmer and the Government are many and warrant financial and technical support. Chapters 4 to 7 review factors that affect farm operations. As-built irrigation systems were adequately designed, but their management require improvements to better match crop water requirements with existing water supplies as these cannot be realistically changed. Such improvements will reduce the running costs of irrigation systems. The best and most useful moisture monitoring techniques are those with predictive elements such as the neutron probe. Irrigating farmers should aim at high water utilization efficiency because of the increasing demand for and cost of water. Management decisions on crop rotations will improve economic returns from the farm and will keep the soil in good heart. Given the condition of limited water supply and unlimited available land area, the seasonal depth of water allocated to each land unit automatically determines the area of land that can be irrigated. Knowledge of dominant crop and environmental conditions is required to obtain accurate estimates of ETcrop for a specific crop. Meteorological conditions determine the evaporative demand while the crop canopy and soil conditions determine the extent to which that demand can be met. Evaporation for a particular crop can be estimated if measurements or estimates of ET0 are available. Knowledge of atmospheric demands (ET0 against ETcrop) and soil moisture status is an important mechanism for scheduling. Many meteorological methods are available but require local calibration for accurate crop water requirement estimations. The Priestley-Taylor method, using locally developed constants (a = 1.35; y = 0.65) was favoured in this study. Many irrigation timing and application options are available to suit the various methods of water application, the physical, economic and labour constraints and the objectives of the farm operations. Existing water storage capacities can meet the full supplemental water requirements for most of the common crop rotations at the scheme. Original irrigation schedules resulted in up to 13.6 % yield reduction from optimum. Least irrigation requirements are obtained when irrigated at 100% depletion of the readily available moisture, refilling the soil below the field capacity to benefit from rain if it occurred after an irrigation was made. Proper timing of the last irrigation will enable high moisture deficits at harvest and minimise water and energy losses.
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