A Hydrological Drought Index for the Clutha Catchment

The Clutha River is New Zealand’s largest river, both in terms of discharge and catchment size. It has a large seasonal variability with large spring and summer flows and very low flows in winter. The river is important for its contribution to energy production from two large hydro dams, irrigation for agriculture and horticulture, and tourist activities. The mountainous South Island topography to the west of the catchment area means that a large part of the catchment is in a rain shadow, and is regularly subject to long periods of time without any significant precipitation. The actual river flows are modified by discretionary releases of water for the only storage lake in the catchment at Hawea.

While floods can be mitigated against, droughts are insidious and generally well established before being identified. A hydrological drought is defined as a lack of water in the hydrological system, identified by abnormally low streamflow in rivers and abnormally low levels in lakes, reservoirs. A drought index is proposed for the upper and mid Clutha Catchment that identifies the onset of hydrological drought conditions, measure the severity of the drought, and signals the end of the drought. This index has the advantage over other hydrological indices in that it uses a daily threshold rather than a monthly or seasonal one, and indicates the actual start of a drought as soon as it occurs.