What causes variability in surface and groundwater in a small catchment?
Dr Hilary McMillan, National Institute of Water and Atmospheric Research, Christchurch, New Zealand.
Even in a small catchment, much of the water that ends up in the river has seeped into the soil and travelled via groundwater pathways. Therefore, to understand the hydrology of a catchment, and ultimately to represent it in a model which predicts river flow, a combined understanding of both the surfacewater and groundwater systems are needed. We also know that both surface (or soil) water and groundwater are highly variable over scales from centimetres to kilometres, and that the distribution of water has a strong influence on the speed and quantity of water flow into the river.
This presentation will describe a study where we extensively instrumented an upland headwater catchment in New Zealand. Our aim was to jointly measure the spatial variation in soil and groundwater quantity, and how it changed over time. We investigated the types of variability occurring, and what physical characteristics control them. Our results showed several distinct causes of variability in water stores. Some types of variability involve a response of only part of the catchment to a rainfall event, e.g. partial response of deep soil moisture in the summer months. Other types of variability involve differences in dynamics, e.g. different parts of the catchment drying out or wetting up at different rates. We also saw an unusual split between slow and fast groundwater responses in different places. I will discuss the implications of the many types of variability we measured, for prediction of river flow in this catchment and further afield.