Moench Allen F
U.S. Geological Survey, Menlo Park, CA 94025, USA.
Ground Water. 2004 Mar-Apr;42(2):223-33. doi: 10.1111/j.1745-6584.2004.tb02669.x.
Analytical models commonly used to interpret unconfined aquifer tests have been based on upper-boundary (water table) conditions that do not adequately address effects of time-varying drainage from the vadose zone. As a result, measured and simulated drawdown data may not agree and hydraulic parameters may be inaccurately estimated. A 72-hour aquifer test conducted in Cape Cod, Massachusetts, in a slightly heterogeneous, coarse-grained, glacial outwash deposit was found to be a good candidate for testing models with different upper-boundary conditions. In general, under the commonly invoked assumption of instantaneous drainage, measured and simulated drawdowns were found to agree with one another only at late time and early time. In the intermediate-time range, because of delayed drainage, measured drawdowns always exceeded simulated values, most noticeably in piezometers located near the water table. To reduce these discrepancies, an analytical model was developed that can fully account for time-varying drainage given that the aquifer is not strongly heterogeneous. The approach is flexible as the model, which makes use of empirical relations, does not constrain drainage to follow any particular functional relation. By this approach, measured and simulated drawdowns agree over the complete time range, and the estimated parameters are consistent with prior studies and with what is known about the aquifer geometry, stratigraphy, and composition. By properly accounting for vadose zone drainage, it was found that realistic estimates of all hydraulic parameters, including specific yield, could be obtained with or without the use of late-time data.
常用于解释无压含水层试验的分析模型一直基于上边界（地下水位）条件，而这些条件未能充分考虑包气带随时间变化的排水影响。因此，实测和模拟的水位下降数据可能不一致，水力参数也可能估计不准确。在马萨诸塞州科德角的一个轻度非均质、粗粒冰川冲积层中进行的一次72小时含水层试验，被发现是测试具有不同上边界条件模型的理想候选对象。一般来说，在通常采用的瞬时排水假设下，实测和模拟的水位下降仅在后期和早期相互吻合。在中间时间段，由于排水延迟，实测水位下降总是超过模拟值，在靠近地下水位的测压管中最为明显。为了减少这些差异，开发了一个分析模型，该模型在含水层并非强非均质的情况下能够充分考虑随时间变化的排水情况。该方法具有灵活性，因为利用经验关系的模型并不限制排水遵循任何特定的函数关系。通过这种方法，实测和模拟的水位下降在整个时间范围内都能吻合，并且估计的参数与先前的研究以及已知的含水层几何形状、地层和成分一致。通过适当考虑包气带排水，发现无论是否使用后期数据，都可以获得包括给水度在内的所有水力参数的实际估计值。
