Measurement Of Losses From On-Farm Channels And Drains

Abstract

Water use and groundwater rises are two important problems facing irrigators in the southern irrigated region of NSW. Water loss through percolation has been extensively studied for irrigated farms and major supply channels but there have been no studies of loss through on-farm channels and drains. This study aimed to determine the magnitude of percolation losses attributable to on-farm channels and drains. It also aimed to consider approaches and for need to identify problem were and to consider likely remediation techniques. Investigations were carried out within selected farms in Coleambally and Murrumbidgee Irrigation Areas in southern NSW during the irrigation seasons of 1997/98, 1998/99 and 1999/00. The Idaho Seepage Meter was used to make point infiltration measurements. Seepage losses were not estimated for all channels on every farm but for only those channels being used by the landholder in the periods seepage measurements were taken. Only 3 of the 9 farms investigated were using all of the channels and drains on the farm during monitoring activities. Only permanent channel and drain structures on the farm were measured. This potentially causes the annual seepage losses calculated per farm to be underestimated. The Idaho Seepage Meter was used for this investigation. Measuring seepage using the Idaho Seepage Meter is rapid, direct, and cheap. The success of this method depends upon the high degree of homogeneity in natural soils. Tests using seepage meters can be conducted in channels without interfering with their normal water delivery operation. Three or four measurements were taken across channels and drains at intervals of 50 - 100 metres. Due to the age and lack of maintenance of these channels their cross-sections had deteriorated. In these situations it was difficult to place the Idaho Seepage Meter on the sides of the channels. Extreme care was taken to cause minimal disturbance to the local soil so that the seepage pattern would not be appreciably affected. A series of test wells was drilled adjacent to the irrigation channels and drains using information obtained from EM 31 electromagnetic surveys. The Idaho Seepage Meter can be used to rapidly locate channel sections with high seepage losses. This enables total seepage losses from a section of channel to be estimated economically. The seepage results from 15-30 year old channels indicate that a combination of weeds and sediment deposition may be the major factor for reduction of seepage. Silt sediments were deposited on the channel bed, or on only part of the bed where the channel was curved, not the entire wetted perimeter of the channel. However, some old channels, which were cleaned prior to the irrigation season, had significant seepage volumes. In new channels and drains low seepage rates were also found at many sites. These sites could have been influenced by factors such as compaction beneath the bed of channel, soil sodicity, biological activity, and slope/bend of channel, and silt deposition in the channel bed. EM-31 surveys were used to characterize soil differences along channel and drainage lines and the apparent electrical conductivity (ECa) values were compared with direct seepage measurements using an Idaho Seepage Meter. This method proved successful in identifying actual seepage sites. ECa values obtained from the EM-31 surveys provided an insight into the most likely locations to have high seepage rates. The EM-31 method was shown to be an important initial predictive tool. Highest seepage rates were found where ECa values were low. In some areas seepage rates were found to be low despite low ECa values at these locations. These anomalies were attributed to various factors which included compaction of substrate, clay layers below channel bed, sodicity, biological activity and sediment deposition. Combining the seepage-monitoring program with the EM31 electromagnetic survey method proved to be highly effective in detailing the nature and extent of the problem. Despite its limitations, the EM31 method is considered to be an important predictive tool in the first stage of loss assessment. Investigation was carried out with in selected farms to quantifying seepage losses from sections of channels and drains in the Murrumbidgee Irrigation Area and Coleambally Irrigation Area. This study enabled those sections with high seepage rates to be identified. As the price of water becoming a realistic figure in terms of meeting actual cost, water distribution efficiencies are being very closely examined. Water loss through seepage in on-farm channels and drains is one of the many elements of the system under investigation. Detection and accurate measurement of seepage is important for the efficient and effective management of on-farm water. However, it is necessary to firstly determine whether a problem exists, and secondly to quantify the extent and seriousness of that problem, before putting resources into seepage control. At high seepage rates it is uneconomical to apply the various treatments available. It is suggested that only methods compatible with and complementary to the natural sealing process will be successful. Lining a leaky channel site will not always completely eliminate seepage losses. In fact, all that can be reasonably expected is a reduction in the seepage rate. The amount of reduction will depend upon the lining used and the magnitude of the loss prior to lining. Investigation sites were established within irrigation channels and drains on each of nine farms, each with an average of approximately 3-4km of unlined on-farm channels carrying water within the farm boundaries and about 1-3km of drains that are used for recycling runoff water.