No abstract

Despite the widespread acceptance of hydrologic importance, controls on the initiation of preferential flow in natural soil profiles and the frequency of its occurrence at different times of year remain elusive. This study determined the controls and frequency of preferential flow occurrence in the Shale Hills Critical Zone Observatory. Soil moisture profiles and precipitation were monitored at 10 sites along a topographic gradient for >3 yr, encompassing 175 precipitation events. For each event and each site, the flow regime was classified as either preferential flow, sequential flow, or nondetectable flow based on the sequence of soil moisture response at various depths within the same site. Preferential flow here specifically refers to out‐of‐sequence soil moisture response, with a deeper horizon responding to precipitation earlier than a shallower horizon. Indices describing antecedent precipitation, precipitation characteristics, precipitation timing, and initial soil moisture were examined to determine the characteristics of events that resulted in preferential flow vs. those that resulted in sequential flow. Analyses showed that preferential flow was common throughout the catchment, occurring during 17 to 54% of the 175 events at each of the 10 monitored sites. Preferential flow occurred in at least one site during 90% of the 175 events. While the frequency of preferential flow appeared insensitive to topographic position, the controls on preferential flow initiation varied with landscape position. Analysis of subsets of the time series data showed that while the frequency of preferential flow can be determined from 1 yr of real‐time monitoring, the controls on preferential flow require much longer (≥3 yr) monitoring to be reliably identified.

Anticipated increases in precipitation intensity due to climate change may affect hydrological controls on soil N 2 O fluxes, resulting in a feedback between climate change and soil greenhouse gas emissions. We evaluated soil hydrologic controls on N 2 O emissions during experimental water table fluctuations in large, intact soil columns amended with 100 kg ha À1 -KNO 3 -N. Soil columns were collected from three landscape positions that vary in hydrological and biogeochemical properties (N 5 12 columns). We flooded columns from bottom to surface to simulate water table fluctuations that are typical for this site, and expected to increase given future climate change scenarios. After the soil was saturated to the surface, we allowed the columns to drain freely while monitoring volumetric soil water content, matric potential and N 2 O emissions over 96 h. Across all landscape positions and replicate soil columns, there was a positive linear relationship between total soil N and the log of cumulative N 2 O emissions (r 2 5 0.47; P 5 0.013). Within individual soil columns, N 2 O flux was a Gaussian function of water-filled pore space (WFPS) during drainage (mean r 2 5 0.90). However, instantaneous maximum N 2 O flux rates did not occur at a consistent WFPS, ranging from 63% to 98% WFPS across landscape positions and replicate soil columns. In contrast, instantaneous maximum N 2 O flux rates occurred within a narrow range (À1.88 to À4.48 kPa) of soil matric potential that approximated field capacity. The relatively consistent relationship between maximum N 2 O flux rates and matric potential indicates that water filled pore size is an important factor affecting soil N 2 O fluxes. These data demonstrate that matric potential is the strongest predictor of the timing of N 2 O fluxes across soils that differ in texture, structure and bulk density.

Diel fluctuations can comprise a significant portion of summer discharge in small to medium catchments. The source of these signals and the manner in which they are propagated to stream gauging sites is poorly understood. In this work, we analysed stream discharge from 15 subcatchments in Dry Creek, Idaho, Reynolds Creek, Idaho, and HJ Andrews, Oregon. We identified diel signals in summer low flow, determined the lag between diel signals and evapotranspiration demand and identified seasonal trends in the evolution of the lag at each site. The lag between vegetation water use and streamflow response increases throughout summer at each subcatchment, with the rate of increase as a function of catchment stream length and other catchment characteristics such as geology, vegetation and stream geomorphology. These findings support the hypothesis that variations in stream velocity are the key control on the seasonal evolution of the observed lags. Copyright © 2012 John Wiley & Sons, Ltd.

Abstract:Deep seepage is a term in the hillslope and catchment water balance that is rarely measured and usually relegated to a residual in the water balance equation. While recent studies have begun to quantify this important component, we still lack understanding of how deep seepage varies from hillslope to catchment scales and how much uncertainty surrounds its quantification within the overall water balance. Here, we report on a hillslope water balance study from the H. J. Andrews Experimental Forest in Oregon aimed at quantifying the deep seepage component where we irrigated a 172-m 2 section of hillslope for 24Ð4 days at 3Ð6 š 3 mm/h. The objective of this experiment was to close the water balance, identifying the relative partitioning of, and uncertainties around deep seepage and the other measured water balance components of evaporation, transpiration, lateral subsurface flow, bedrock return flow and fluxes into and out of soil profile storage. We then used this information to determine how the quantification of individual water balance components improves our understanding of key hillslope processes and how uncertainties in individual measurements propagate through the functional uses of the measurements into water balance components (i.e. meteorological measurements propagated through potential evapotranspiration estimates). Our results show that hillslope scale deep seepage composed of 27 š 17% of applied water. During and immediately after the irrigation experiment, a significant amount of the irrigation water could not be accounted for. This amount decreased as the measurement time increased, declining from 28 š 16% at the end of the irrigation to 20 š 21% after 10 days drainage. This water is attributed to deep seepage at the catchment scale.