A quasi‐dynamic wetness index that accounts for variable drainage times since a prior rainfall event is derived from simple subsurface flow theory. The method is tested through a series of field observations and numerical experiments using a spatially distributed, dynamic hydrologic model. The quasi‐dynamic wetness index is shown to be a useful extension of previously developed static indices for predicting the location of zones of soil saturation and the distribution of soil water (i.e., the soil water content overlying a shallow impermeable or semiimpermeable layer). The new index is not constrained by the steady state assumption that forms the basis of existing indices.
/ A buffer strip can perform a multitude of functions, and these include channel stability, a filter for sediment and nutrients, water purification (e.g., bacteria and pathogens), a nondisturbance area, and the provision of terrestrial and stream habitat. These functions are reviewed with specific application to Australian conditions, and methods for modeling their performance are outlined. The primary focus is on the use of buffer strips to minimize waterway pollution from diffuse sources since their use is often justified on this basis. Particular attention is given to the conditions under which a buffer strip will act as an effective filter and the conditions under which it will fail. Buffer strips are most effective when the flow is shallow (nonsubmerged), slow, and enters the buffer strip uniformly along its length. Their sediment trapping performance decreases as the sediment particle size decreases. Nutrients are often preferentially attached to fine sediment. As a result, buffer strips are better filters of sediment than of nutrients. Buffer strips should only be considered as a secondary conservation practice after controlling the generation of pollutants at their source and, to be effective, buffer strips should always be carefully designed, installed, and maintained.The term buffer strip means different things to different people. In agricultural and some forestry operations a buffer strip normally implies a strip of vegetation that acts as a filter for sediment and their attached nutrients and pollutants. In this way it improves or maintains the quality of water further downslope. Some view a buffer strip as the wetlands that delay and purify water adjacent to rivers and streams, while others think of it as the riparian zone (i.e., situated along the banks of a river or stream) that influences bank and channel stability and which has a primary role in determining the structure and function of the stream habitat. Each of these examples involves a mixture of physical and biological processes and often multiple functions. In determining if a buffer strip is an appropriate management strategy for a given problem, the following five questions need to be answered: (1) What are the processes involved? (2) What variety of functions can a vegetative strip serve? (3) How well does it perform these functions? (4) How often (i.e., when)? (5) Exactly where does it need to perform these functions? The physical processes that are involved vary, depending on a wide KEY WORDS: Buffer strips; Vegetative filter strips; Sediment; Nutrients; Waterway pollution *Author to whom correspondence should be addressed.range of factors including soil type, vegetation, land use, rainfall intensity and duration, and location within a landscape. This paper examines these questions. While a buffer strip can perform a multitude of functions including channel stability, a filter for sediment and nutrients, water purification (e.g., bacteria and pathogens), a nondisturbance area, and the provision of terrestrial and stream habit...
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