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Development and application of a technique for evaluating root zone drainage Willington, Robert Peter

Abstract

An understanding of the water balance of forest ecosystems is of benefit to foresters who manage second rotation stands. Maximizing the mean annual increment can be better realized if recognition is given to the importance of the water balance. One component of the water balance, root zone drainage, is not only important, but also difficult either to measure or calculate. In this study, root zone drainage during a two-week drying cycle was monitored by a method based on Darcy's Law. To implement the method, a tensiometer-pressure transducer system was developed which constantly monitored the soil water potential at five soil depths. The system was easily installed, required minimum maintenance and measured the potential of the soil water to within ± 1.8 cm. H₂O with a response time of less than five seconds. The cumulative root zone drainage for the two-week period was 11.72 mm. with an estimated maximum error of the daily drainage of ± 1.30 mm./day. Comparable root zone drainage derived from an independent but simultaneous evaluation by the water balance method was 9.33 mm. with an estimated maximum error of the daily drainage of ± 2.kl mm./day. Both positive and negative drainage to the root zone was observed. The negative drainage (water moving into the root zone) provides a supplementary supply of water to the root zone, thereby reducing the possibility of a soil water deficit occurring.

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