Vertical Changes in Soil Moisture in a Tussock Grassland Soil

Abstract

Long term, continuous field based studies on soil moisture in New Zealand are scarce, with those addressing the fine scale and providing continuous measurement even more so. The tussock grasslands of Otago have been the focus of significant previous research, which has highlighted the distinctive water use of these plants. Tussock grasses are particularly efficient in terms of water use, having low transpiration and water uptake rates, as well as providing good cover of the soil surface from solar radiation, reducing evaporation rates. These factors combined with the high annual rainfall which occurs in the high elevation regions of Otago lead to the importance of tussock vegetation on downstream hydrology. However, there is a distinct lack of understanding of the behaviour of soil water with depth within soil stratum vegetated by tussock grasses. The aim of this project is to directly measure the vertical movement of soil moisture. A field site within the Glendhu experimental catchments was selected as the location of a 12 month in situ measurement of climatic variables and soil water. An array of six time domain reflectometry probes and two capacitance probes, as well as an automatic weather station were installed at the field site, culminating in a 12 month, continuous record of soil moisture. Laboratory methods including bulk density and particle size analysis have been utilized to complement the field instrumentation. Overall, the annual dataset described a cool and wet year, with consistently wet soil conditions, particularly through the upper soil strata. There was a clear seasonality within the soil moisture, with elevated winter values indicating winter recharge of soil water reservoirs. Responses within the soil strata were typified as typical or atypical, with the atypical responses categorised by a lack of organisation within the response. Typical events were characterised as a pattern of general vertical increase, with typical lag times of six hours at 10 cm, seven at 20 cm, eight at 30 cm and 13.5 hours at 42 cm. Time domain reflectometry probes were found to be the most accurate in this type of research, and are the instrument of choice for future research. This project has comprehensive measurement of soil moisture attenuation with depth carried out in this project will allow for the development of a more comprehensive understanding of the soil moisture system at the site, as well as providing data to consider the adequacy of measuring soil moisture exclusively in the surface zone.