Abstract

Understanding the functioning of the Critical Zone requires intensive measurements at the watershed scale. A common yet rarely explored question in field measurements is: what, when, and where should the measurements be made? For hydrologic processes, stream discharge (Q) and point-based soil moisture (SM) have been commonly monitored. The recent advent of area-based Cosmic-ray Soil Moisture Observing System (COSMOS) SM measurement (with a footprint of ~ 300 m) has provided an additional choice. We hypothesize that the critical measurements for hydrologic processes should occur at times and locations where key water dynamics can be recorded at the temporal and spatial scales of interests. The hypothesis is tested by assessing the three hydrologic measurements, namely, Q, point SM by frequency domain reflectometry (FDR), and area-averaged COSMOS SM, in representing hydrologic responses and in constraining the hydrologic model Flux-PIHM at the Garner Run (GR) catchment in SSHCZO. Both the FDR and COSMOS SM data reflect the transient responses to external forcings, i.e., the pulses of rainfall events and the largest recession in summer under high solar radiation and vegetation uptake. The COSMOS has a less flashy response than FDR due to its large footprint showing a better representativeness at the catchment scale behavior; it also measures a higher SM during the winter because of its representation of both liquid and frozen water. The measured FDR SM at the hillslope and the COSMOS SM are consistent with the Flux-PIHM simulated total water storage dynamics at the whole watershed, whereas the valley floor FDR shows more transient balancing between influx from the upland and out fluxes (Q and ET). In constraining the hydrologic model, Q measurements are critical. The dynamics of SM at the valley floor captures the combined responses of multiple processes in the watershed, which makes it an important location for measurement. Compared to the FDR, COSMOS shows less dynamics due to the aggregating effects at a larger footprint; it also records the large water content in the vegetation during the summer, which masks the response of soil moisture to transient precipitation events. Both factors make it a not-as-important constraint for the hydrologic model however it reflects better the hydrologic states of the catchment.

Citation

Dacheng Xiao*, Yuning Shi, Qicheng Tang, Henry Lin, David M Eissenstat, Susan Brantley, Li Li (2018): Critical measurements of hydrologic response in a first-order, forested catchment at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO). Abstract EP11C-2065 presented at 2018 AGU Fall Meeting, Washington, D.C., 10-14 Dec.

This Paper/Book acknowledges NSF CZO grant support.