On the Sensitivity of Hillslope Runoff and Channel Transmission Losses in Arid Piedmont Slopes

Abstract

Channel transmission losses alter the streamflow response of arid and semiarid watersheds and promote focused groundwater recharge. This process has been primarily studied in dryland channels draining large areas that are displaced away from hillslope runoff generation. In contrast, small watersheds on arid piedmont slopes allow the investigation of interactive hillslope and channel processes that control the partitioning between surface and subsurface flows. In this study, we utilize high‐resolution, long‐term measurements of water balance components in an instrumented watershed of the Chihuahuan Desert to set up, parameterize, and test a process‐based, distributed hydrologic model modified to account for channel losses. A transient method for capturing capillary effects in channels results in simulations with a reliable representation of the watershed energy balance, soil moisture dynamics, hillslope infiltration, channel transmission (or percolation) losses, and streamflow yield over the study period. The simulation also reproduces a conceptual model of hillslope infiltration‐excess runoff generation linked to downstream channel percolation losses that depend on the rainfall event size. Model‐derived thresholds were obtained for the amount of hillslope runoff (6 mm) and rainfall (12.5 mm) necessary for streamflow yield, such that 40% of percolation occurs for small events that do not reach the outlet. Using a set of scenarios, we identify that hillslope infiltration controls the rainfall threshold necessary to initiate percolation, while channel infiltration affects the partitioning into percolation and streamflow yield. Thus, the connectivity along hillslope‐channel pathways is deemed an essential control on the streamflow generation and groundwater recharge in arid regions with complex terrain.

Extreme weather events and transmission losses in arid streams

“…Changes to erosion in the future may also impact transmission losses by impacting channel geometry and sediment properties. For a discussion of the effect of changes in channel hydraulic conductivity, the reader is referred to Schreiner-McGraw and Vivoni (2018). Due to constant channel geometry in our simulations (and therefore constant channel storage capacity), ephemeral streamflow is more sensitive to extreme precipitation events than transmission losses.…”

A new approach for measuring surface hydrological connectivity

“…Moreover, the assumed vertical progression of the wetting front cannot be guaranteed (Sanders, 1998); lateral spreading may again lead to an overestimation of infiltration ( Figure 1b). This final limitation is partially overcome by the use of a double ring infiltrometer, where a second outer ring is inserted deeper into the soil, and the space between the rings is filled with water prior to the start of the test (Figure 1c; Al-Awadhi, 2013; Guzha, 2004;Perrolf & Sandstrom, 1995;Verbist, Cornelis, Torfs, & Gabriels, 2013).…”

“…By gradually increasing the tension or negative pressure head of the water supply reservoir ( Figure 1d), increasingly smaller pore sizes (starting from the largest) are excluded from conducting water into the soil (Brady & Weil, 2008), thereby permitting direct comparison of infiltration rates for F I G U R E 1 Infiltration methods commonly used in drylands. (a) rainfall simulator, (b) single-ring infiltrometer (adapted from Sanders, 1998), (c) double-ring infiltrometer (adapted from Sanders, 1998), (d) tension infiltrometer (adapted from Amoozegar & Wilson, 1999), and (e) Minidisk tension infiltrometer (adapted from Decagon Devices, 2016) different soil pore sizes (Kelishadi, Mosaddeghi, Hajabbasi, & Ayoubi, 2014;Verbist et al, 2013;Young, McDonald, Caldwell, Benner, & Meadows, 2004;Zhou, Hu, Cheng, Wang, & Li, 2011). Although tests can run for longer than ring infiltrometers, measurements can be automated to reduce the burden of field time and enable multiple simultaneous tests (Ankeny, Kaspar, & Horton, 1988).…”

“…The degree to which isolated surface runoff generating areas are connected to the drainage network is a key determinant of flood magnitude, yet we know little about the dynamics of this connection process. Although much has been done to improve our conceptualization of this functional (or process based) connectivity (Bracken et al, 2013;Kirkby, Bracken, & Reaney, 2002;Reaney, Bracken, & Kirkby, 2007;Reaney, Bracken, & Kirkby, 2014;Schreiner-McGraw & Vivoni, 2018;Turnbull, Wainwright, & Brazier, 2008), we still know little about the real-world operation of the processes making and maintaining the connections. Downslope water fluxes via overland flows must overcome a combination of flow resistance and downslope transmission losses to reach an outlet (Mueller, Wainwright, & Parsons, 2007;Smith et al, 2010).…”

“…Downslope water fluxes via overland flows must overcome a combination of flow resistance and downslope transmission losses to reach an outlet (Mueller, Wainwright, & Parsons, 2007;Smith et al, 2010). Although much has been done to improve our conceptualization of this functional (or process based) connectivity (Bracken et al, 2013;Kirkby, Bracken, & Reaney, 2002;Reaney, Bracken, & Kirkby, 2007;Reaney, Bracken, & Kirkby, 2014;Schreiner-McGraw & Vivoni, 2018;Turnbull, Wainwright, & Brazier, 2008), we still know little about the real-world operation of the processes making and maintaining the connections.…”