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0 <br />ii <br />E <br />Ll <br />Ci <br />u <br />F <br />r <br />1-1 <br />L <br />P <br />L <br />C <br />H <br />r] <br />Drain Gauge User's Manual <br />3. Theory <br />Divergence Control Tube Length <br />Optimal performance of traditional passive capillary <br />lysimeters is only achieved by precisely matching wick <br />length to soil type. The addition of an innovative diver- <br />gence control tube (DCT) negates the need for precise <br />wick -soil matching in the Drain Gauge. Numerical and <br />laboratory simulations performed by Gee et al. (2002) <br />have demonstrated the effectiveness of the DCT in pre- <br />venting flux divergence around the collection point. Fig- <br />ure 5 shows the collection effectiveness of the drain gauge <br />as a function of DCT height for six different flux rates in a <br />coarse sand. It is apparent that in this medium, reasonable <br />collection effectiveness can be achieved with a DCT <br />height of as little as 30 cm, even at very low drainage <br />fluxes where divergence most readily occurs. For finer tex- <br />tured soils, a DCT height of up to 60 cm may be neces- <br />sary. In the extreme case of very fine soils with low <br />drainage fluxes (i.e. <50 mm yf 1), a DCT height of more <br />than 60 cm may be necessary to prevent flow divergence. <br />1.2 <br />1.0 <br />0.8 <br />0.6 <br />OA <br />0.2 <br />0.0 <br />100 101 102 <br />Heigh of Divergence Barrier (cm) <br />Figure 5: Collection effectiveness of Drain Gauge for several different flux <br />rates as a function of DCT height in a coarse sand. JA is the actual drain- <br />age flux, andj,, is the drainage flux measured by the Drain Gauge. Fig- <br />ure adapted from Gee et al. (2002). <br />17 <br />