Laserfiche WebLink
Ci <br />Li <br />u <br />i <br />u <br />F <br />Drain Gauge User's Manual <br />3. Theory <br />sured and accounted for. Because of uncertainties in the <br />measurements of the other water balance components, <br />deep drainage estimates were subject to large errors. <br />The Drain Gauge now allows direct measurement of the <br />deep drainage component of the water balance. This is <br />accomplished by intercepting and collecting a representa- <br />tive sample of the water that moves below the root zone. <br />The Drain Gauge is sometimes referred to as a passive <br />wick lysimeter. It has a specially treated fiberglass wick <br />which maintains a tension on the water at the bottom of <br />the soil profile where it is extracting water. Without this <br />tension, water would "pile up" at the outflow boundary, <br />and force the water in the soil above to move around the <br />Drain Gauge, rather than into it. The divergence control <br />tube on the top of the Drain Gauge is also for the purpose <br />of maintaining vertical flow above the Gauge so that the <br />Drain Gauge intercepts a representative sample. <br />Both the amount of sample and its chemical composition <br />need to be representative of deep drainage in the area <br />where the drain gauge is installed. Whether or not this is <br />true is determined largely by the installation. The main <br />issues are: <br />• Depth of installation <br />• Soil disturbance above the Drain Gauge. <br />• Contact between the wick and the soil profile. <br />• Root incursion into the Drain Gauge wick section. <br />Following -are some general comments related to these <br />issues. Specific installation procedures are then outlined in <br />the next chapter. <br />15 <br />