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Drain Gauge User's Manual <br />3. Theory <br />-•,1 • •'' • •i <br />In any soil profile, there is a zone of recharge and seasonal <br />depletion, extending to the bottom of the root zone. Since <br />root density decreases with depth, the bottom of this zone <br />may be difficult to locate. In annual crops it is typically <br />around 1 in (3 ft.), but can be much deeper in perennials. <br />There is, of course, a practical limit to how deep the Drain <br />Gauge can be installed. With shallow -rooted crops this is <br />not an issue, but with deep-rooted plants, one may need to <br />strike a compromise between getting below all roots and <br />installing the Drain Gauge at a practical depth. Even when <br />roots go quite deep, the amount of water taken up by <br />these roots may be quite small. <br />Another consideration is the depth of the water table. The <br />water table depth must always be below the bottom of <br />the Drain Gauge so that water can drain out. This <br />might sometimes require that the Drain Gauge be <br />installed at a shallower depth than would be ideal to <br />accommodate the root zone. <br />The consequence of installing the Drain Gauge at too <br />deep a depth is that it will require a long time to come to <br />steady state and give drainage numbers that are represen- <br />tative. The consequence of installing it at too shallow a <br />depth is an over -estimation of deep drainage because <br />some water that would have been transpired is intercepted <br />and measured as drainage. It is also possible that roots <br />could grow down the wick and use water from, or clog the <br />Drain Gauge. This can be minimized by the Treflan - <br />impregnated root inhibitor fabric at the bottom of the <br />divergence control tube (installed). <br />16 <br />