A look inside ‘black box’ hydrograph separation models: a study at the Hydrohill catchment

Abstract

Abstract:Runoff sources and dominant flowpaths are still poorly understood in most catchments; consequently, most hydrograph separations are essentially 'black box' models where only external information is used. The well-instrumented 490 m 2 Hydrohill artificial grassland catchment located near Nanjing (China) was used to examine internal catchment processes. Since groundwater levels never reach the soil surface at this site, two physically distinct flowpaths can unambiguously be defined: surface and subsurface runoff. This study combines hydrometric, isotopic and geochemical approaches to investigating the relations between the chloride, silica, and oxygen isotopic compositions of subsurface waters and rainfall.During a 120 mm storm over a 24 h period in 1989, 55% of event water input infiltrated and added to soil water storage; the remainder ran off as infiltration-excess overland flow. Only about 3-5% of the pre-event water was displaced out of the catchment by in-storm rainfall. About 80% of the total flow was quickflow, and 10% of the total flow was pre-event water, mostly derived from saturated flow from deeper soils. Rain water with high υ 18 O values from the beginning of the storm appeared to be preferentially stored in shallow soils. Groundwater at the end of the storm shows a wide range of isotopic and chemical compositions, primarily reflecting the heterogeneous distribution of the new and mixed pore waters. High chloride and silica concentrations in quickflow runoff derived from event water indicate that these species are not suitable conservative tracers of either water sources or flowpaths in this catchment. Determining the proportion of event water alone does not constrain the possible hydrologic mechanisms sufficiently to distinguish subsurface and surface flowpaths uniquely, even in this highly controlled artificial catchment. We reconcile these findings with a perceptual model of stormflow sources and flowpaths that explicitly accounts for water, isotopic, and chemical mass balance.

Incipient subsurface heterogeneity and its effect on overland flow generation – insight from a modeling study of the first experiment at the Biosphere 2 Landscape Evolution Observatory

“…Experimental facilities that can tackle the evolution of structure and function in physical and biological systems, along with their emergent processes at scale, will be extremely useful for understanding future Earth system states and the significant deviation from stationarity seen in our current climate system. Unlike other artificial laboratories such as the Hydrohill in China (Kendall et al, 2001) and the Chicken Creek in Germany (Gerwin et al, 2009;Hofer et al, 2011), LEO was built with homogeneous soil and with a focus on evolving heterogeneity from a "time-zero" homogeneous condition through co-evolution of the soil-water-biota system over a timescale of years (Hopp et al, 2009;Dontsova et al, 2009). Development of catchment morphology and soil catena driven by hydrological processes through soil erosion and deposition may be one of the major causes that induce heterogeneity and that in turn exert strong feedbacks on hydrological processes (e.g., Beven et al, 1988;Sivapalan, 2005;McDonnell et al, 2007;Troch et al, 2009).…”

Macropore flow of old water revisited: experimental insights from a tile-drained hillslope

“…There have been numerous studies that show two component source fractions exceeding 100 % (e.g. Swistock et al, 1989), and several studies have shown the importance of vadose zone contributions to storm runoff by sampling the soil water end-member and quantifying its effect on the hydrograph composition in the context of hydrograph separation (Kennedy et al, 1986;Swistock et al, 1989;Bazemore et al, 1994;Kendall et al, 2001). In theory, the use of a threecomponent hydrograph separation, accounting for soil water contributions, can help to overcome this (e.g.…”

A distributed stream temperature model using high resolution temperature observations

“…Often isotopes such as deuterium (Wenninger et al, 2004), tritium or oxygen-18 (Sklash and Farvolden, 1979;Uhlenbrook and Leibundgut, 2002) are used to distinguish between event water and pre-event water. Besides these, dissolved silica and major ions (such as Cl − , Na + , K + , Ca + and Mg + ) have been used, for a two, three or even a five component separation (Katsuyama et al, 2001;Kendall et al, 2001;Uhlenbrook and Hoeg, 2003).…”