How Many Measurements of Soil Moisture within the Footprint of a Ground‐Based Microwave Radiometer Are Required to Account for Meter‐Scale Spatial Variability?
How Many Measurements of Soil Moisture within the Footprint of a Ground‐Based Microwave Radiometer Are Required to Account for Meter‐Scale Spatial Variability?
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DOI：
10.2136/vzj2012.0100
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发表时间：
2013-08
影响因子：
2.8
通讯作者：
L. Bramer;B. Hornbuckle;Petruta Caragea
中科院分区：
文献类型：
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作者：
L. Bramer;B. Hornbuckle;Petruta Caragea
地面微波遥感测量与土壤湿度的点测量的验证仍然是一项重要的任务。要确定一个面积为几十平方米的区域的平均土壤湿度所需的点测量数量，以及进行这些观测的方式仍然是未知的。我们在爱荷华州中部进行了一项实地实验，以表征土壤水分的空间过程和采样策略，用于估计代表地面微波辐射计足迹的区域的平均土壤水分。我们确定了裸露土壤的水分含量在这个尺度上的空间依赖性，这使得从点测量的平均土壤水分的测定。这种空间依赖性可以用指数变差函数模型来描述。该模型的参数根据平均湿度条件而变化。平均土壤湿度估计的准确性取决于点测量的数量、使用的采样技术和平均湿度条件。当我们使用空间相关性时，我们提高了平均值估计的准确性和精度。当需要更高的平均土壤湿度估计精度时，格点和分层抽样方案比随机抽样需要更少的测量。实际上，如果在随机位置测量土壤湿度，并且不使用空间依赖性，我们发现在7 × 9 m的足迹内进行大约20次测量，估计的平均值接近我们观察到的整个土壤湿度条件范围的真实值的0.01 m3 m−3。
Validation of ground‐based microwave remote sensing measurements with point measurements of soil moisture remains an important task. The number of point measurements needed to characterize the mean soil moisture of an area on the order of tens of square meters, and the manner in which these observations should be made remain unknown. We conducted a field experiment in central Iowa to characterize the spatial process of soil moisture and sampling strategies for estimating the mean soil moisture of an area that represents the footprint of a ground‐based microwave radiometer. We determined the spatial dependence of the moisture content of bare soil at this scale, which allows the determination of the mean soil moisture from point measurements. This spatial dependence can be described by an exponential variogram model. The parameters of this model vary according to mean moisture conditions. The accuracy of the estimate of mean soil moisture depends on the number of point measurements, what sampling technique is used, and mean moisture conditions. When we use spatial dependence, we increase the accuracy and precision of our estimates of the mean. Lattice and stratified sampling schemes require fewer measurements than random sampling when a higher level of accuracy in the estimate of mean soil moisture is desired. Practically, if measurements of soil moisture are made in random locations and spatial dependence is not used, we found that approximately 20 measurements within a 7 by 9 m footprint would estimate the mean to within close to 0.01 m3 m−3 of the true value for the entire range of soil moisture conditions that we observed.