| A hydrologic response unit (HRU) is a unique spatial component of the soil landscape that exhibits hydrologic similarity. This term was originally coined in 1983 by Leavesley and associates. The focus of this study is on understanding the spatial variability of hydrologic response units as they pertain to plant growth. The goal is to improve our understanding of the spatial patterns of soil hydrologic processes that are important for plant growth, and to develop means for indirect measurement of these processes.; Two fields in northern Illinois (i.e., “North” and “South”) were selected for the study. Fuzzy mapping methods were used to delineate the HRUs from the different parameter sets (i.e., direct vs. indirect measurements). The results were then compared to determine their degree of similarity.; The directly measured (“benchmark”) HRU attribute classes are developed from ground water dynamics, the modeled available water holding capacity, and the 1999–2000 soil moisture “environment” at select monitoring locations. The indirectly generated HRU attribute classes are based on soil texture, mollic epipedon thickness, air-dried spectral reflectance, a topographic parameter, and a geophysical parameter from grid- as well as landscape-based sampling. The selected surrogate measurements provided a useful foundation to delineate plant growth HRUs at both study sites, once “leveraging locations” were determined. If the selected surrogate variables can be measured in a soil landscape similar to the study sites, and care is taken to eliminate “leveraging locations,” the surrogate-based plant growth HRUs may be similar to what would be delineated from two years of intensive hydrological measurements.; Overall, the low relief North Farm showed indications of a stronger HRU “signal” than the higher relief South Farm. Evidence of this is found in the higher normalized Dunn's coefficient values for both the surrogate- and hydrologic-based fuzzy HRU delineations. The correlation between the surrogate- and hydrologic-based HRUs is also much higher in the case of the North Farm. This could be the result of the spatial continuity of leveraging soil moisture nests, the installation of drainage tile at the South Farm, or the presence of unusual non-leveraging moisture nests at the South Farm. |
