Measurement Of Losses From On-Farm Channels And Drains

摘要

Water use and groundwater rises are two important problems facing irrigators in theudsouthern irrigated region of NSW. Water loss through percolation has been extensivelyudstudied for irrigated farms and major supply channels but there have been no studies ofudloss through on-farm channels and drains.udThis study aimed to determine the magnitude of percolation losses attributable to on-farmudchannels and drains. It also aimed to consider approaches and for need to identify problemudwere and to consider likely remediation techniques.udInvestigations were carried out within selected farms in Coleambally and MurrumbidgeeudIrrigation Areas in southern NSW during the irrigation seasons of 1997/98, 1998/99 andud1999/00. The Idaho Seepage Meter was used to make point infiltration measurements.udSeepage losses were not estimated for all channels on every farm but for only thoseudchannels being used by the landholder in the periods seepage measurements were taken.udOnly 3 of the 9 farms investigated were using all of the channels and drains on the farmudduring monitoring activities. Only permanent channel and drain structures on the farmudwere measured. This potentially causes the annual seepage losses calculated per farm to beudunderestimated.udThe Idaho Seepage Meter was used for this investigation. Measuring seepage using theudIdaho Seepage Meter is rapid, direct, and cheap. The success of this method depends uponudthe high degree of homogeneity in natural soils. Tests using seepage meters can beudconducted in channels without interfering with their normal water delivery operation.udThree or four measurements were taken across channels and drains at intervals of 50 - 100udmetres. Due to the age and lack of maintenance of these channels their cross-sections haduddeteriorated. In these situations it was difficult to place the Idaho Seepage Meter on theudsides of the channels. Extreme care was taken to cause minimal disturbance to the localudsoil so that the seepage pattern would not be appreciably affected.udA series of test wells was drilled adjacent to the irrigation channels and drains usingudinformation obtained from EM 31 electromagnetic surveys.udThe Idaho Seepage Meter can be used to rapidly locate channel sections with high seepageudlosses. This enables total seepage losses from a section of channel to be estimatedudeconomically.udThe seepage results from 15-30 year old channels indicate that a combination of weeds andudsediment deposition may be the major factor for reduction of seepage. Silt sediments wereuddeposited on the channel bed, or on only part of the bed where the channel was curved, notudthe entire wetted perimeter of the channel. However, some old channels, which wereudcleaned prior to the irrigation season, had significant seepage volumes.udIn new channels and drains low seepage rates were also found at many sites. These sitesudcould have been influenced by factors such as compaction beneath the bed of channel, soiludsodicity, biological activity, and slope/bend of channel, and silt deposition in the channeludbed.udEM-31 surveys were used to characterize soil differences along channel and drainage linesudand the apparent electrical conductivity (ECa) values were compared with direct seepageudmeasurements using an Idaho Seepage Meter. This method proved successful inudidentifying actual seepage sites. ECa values obtained from the EM-31 surveys provided anudinsight into the most likely locations to have high seepage rates. The EM-31 method wasudshown to be an important initial predictive tool.udHighest seepage rates were found where ECa values were low. In some areas seepage ratesudwere found to be low despite low ECa values at these locations. These anomalies wereudattributed to various factors which included compaction of substrate, clay layers belowudchannel bed, sodicity, biological activity and sediment deposition.udCombining the seepage-monitoring program with the EM31 electromagnetic surveyudmethod proved to be highly effective in detailing the nature and extent of the problem.udDespite its limitations, the EM31 method is considered to be an important predictive tooludin the first stage of loss assessment.udInvestigation was carried out with in selected farms to quantifying seepage losses fromudsections of channels and drains in the Murrumbidgee Irrigation Area and ColeamballyudIrrigation Area. This study enabled those sections with high seepage rates to be identified.udAs the price of water becoming a realistic figure in terms of meeting actual cost, wateruddistribution efficiencies are being very closely examined. Water loss through seepage inudon-farm channels and drains is one of the many elements of the system under investigation.udDetection and accurate measurement of seepage is important for the efficient and effectiveudmanagement of on-farm water. However, it is necessary to firstly determine whether audproblem exists, and secondly to quantify the extent and seriousness of that problem, beforeudputting resources into seepage control. At high seepage rates it is uneconomical to applyudthe various treatments available. It is suggested that only methods compatible with andudcomplementary to the natural sealing process will be successful. Lining a leaky channeludsite will not always completely eliminate seepage losses. In fact, all that can be reasonablyudexpected is a reduction in the seepage rate. The amount of reduction will depend upon theudlining used and the magnitude of the loss prior to lining.udInvestigation sites were established within irrigation channels and drains on each of nineudfarms, each with an average of approximately 3-4km of unlined on-farm channels carryingudwater within the farm boundaries and about 1-3km of drains that are used for recyclingudrunoff water.