No abstract

From 30 September to 2 October 1999 a workshop was held in Gif-sur-Yvette, France, with the central objective to develop a research strategy for the next 3-5 years, aiming at a systematic description of root functioning, rooting depth, and root distribution for modeling root water uptake from local and regional to global scales. The goal was to link more closely the weather prediction and climate and hydrological models with ecological and plant physiological information in order to improve the understanding of the impact that root functioning has on the hydrological cycle at various scales. The major outcome of the workshop was a number of recommendations, detailed at the end of this paper, on root water uptake parameterization and modeling and on collection of root and soil hydraulic data.

Abstract:Tracer investigations were combined with a geographical information system (GIS) analysis of the 31 km 2 Girnock catchment (Cairngorm Mountains, Scotland) in order to understand hydrological functioning by identifying dominant runoff sources and estimating mean residence times. The catchment has a complex geology, soil cover and topography. Gran alkalinity was used to demonstrate that catchment geology has a dominant influence on baseflow chemistry, but flow paths originating in acidic horizons in the upper soil profiles controlled stormflow alkalinity. Chemically based hydrograph separations at the catchment scale indicated that ¾30% of annual runoff was derived from groundwater sources. Similar contributions (23-36%) were estimated for virtually all major sub-basins. υ 18 O of precipitation (mean: 9Ð4‰; range: 16Ð1 to 5Ð0‰) and stream waters (mean: 9Ð1‰; range: 11Ð6 to 7Ð4‰) were used to assess mean catchment and sub-basin residence times, which were in the order ¾4-6 months. GIS analysis showed that these tracer-based diagnostic features of catchment functioning were consistent with the landscape organization of the catchment. Soil and HOST (Hydrology of Soil Type) maps indicated that the catchment and individual sub-basins were dominated by hydrologically responsive soils, such as peats (Histosol), peaty gleys (Histic Gleysols) and rankers (Umbric Leptosols and Histosols). Soil cover (in combination with a topographic index) predicted extensive areas of saturation that probably expand during hydrological events, thus providing a high degree of hydrological connectivity between catchment hillslopes and stream channel network. This was validated by aerial photographic interpretation and groundtruthing. These characteristics of hydrological functioning (i.e. dominance of responsive hydrological pathways and short residence times) dictate that the catchment is sensitive to land use change impacts on the quality and quantity of streamflows. It is suggested that such conceptualization of hydrological functioning using tracer-validated GIS analysis can play an important role in the sustainable management of river basins.

The compilation of a database of soil carbon and land use is described, from which models of soil carbon dioxide emissions across the United Kingdom (UK) can be run. The database gives soil organic carbon, sand, silt and clay contents and bulk densities weighted to reference layers from 0 to 30 cm and from 30 to 100 cm depths. The data are interpolated from information on soil types and land use on a 1 km grid across the UK and are used to estimate soil carbon stocks. For 1990, the baseline year for the Kyoto Protocol on carbon emissions, the estimate is 4562 Tg soil organic carbon in the top 1 m of soil across the UK, with an average density of 18 kg m 22 . The data can be reported by layer (e.g. 54% in topsoils) and country (e.g. 48% in Scotland) as well as by soil and land type.

[1] This paper presents a vision that advocates hydropedology as an advantageous integration of pedology and hydrology for studying the intimate relationships between soil, landscape, and hydrology. Landscape water flux is suggested as a unifying precept for hydropedology, through which pedologic and hydrologic expertise can be better integrated. Landscape water flux here encompasses the source, storage, flux, pathway, residence time, availability, and spatiotemporal distribution of water in the root and deep vadose zones within the landscape. After illustrating multiple knowledge gaps that can be addressed by the synergistic integration of pedology and hydrology, we suggest five scientific hypotheses that are critical to advancing hydropedology and enhancing the prediction of landscape water flux. We then present interlinked strategies for achieving the stated vision. It is our hope that by working together, hydrologists and pedologists, along with scientists in related disciplines, can better guide data acquisition, knowledge integration, and model-based prediction so as to advance the hydrologic sciences in the next decade and beyond.