Drainage of flooded water : effects on baseflow in Awanui Stream, New Zealand

Abstract: In the Heretaunga Plains area, New Zealand, parts of the low lying land adjacent to the Awanui Stream are flooded annually. The purpose of the study was to find out if the flooding water trapped in the field gets sealed off from infiltrating the soils in any way (and hence is unavailable to replenish the stream flow). What would be the effects on stream base flow if pumping of the flooding water would occur direct to the stream after wet periods and heavy rains? The method of this project was to investigate the infiltration, soil type and ground water conditions in the field. The infiltration was investigated with the help of a double ring infiltration test, a disc permeameter that measures hydraulic conductivity, and pvc-pipes with core samples were saturated for an extended period of time to find out if there was any kind of seal forming during saturated conditions. The soil in field was sampled and a soil fraction test was performed. The potential evaporation was measured with an evaporation pan and calculated with data from a climate station in field. With flow records from the outgoing drain, potential evaporation and precipitation data a rough water balance model could be created. The results showed that there is no seal formed in the top part of the soil profile preventing the water from infiltrating. The flooding water is the result of a rising groundwater table, on top of a thick clay layer seven meters down in the ground. Once the flooding water has drained and evaporated away there is nothing wrong with the infiltration rate in field. There are very fine particles of silt and clay in the top soil that decreases the infiltration rate and can cause a separation of the ground water and the water above land surface. When the project was finished two recommendations could be given to the landowner to solve the problem with the flooding. The recommendations were to either re-level the field to get the surface water to runoff towards the drains instead of being trapped in the current low parts of the field. Or to dig drains from Horonui Drain and Cambell Drain into the field's low parts and in that way drain the flooding water away.