Soil water balance - a quick guide to droughts.
The soil water balance is a method of calculating how much rain or irrigation water in a given soil depth is used by plants, or is lost through drainage or by surface runoff. The historical climate record enables us to see how the water balance varies across a range of climatic conditions, an aid to evaluating the impact and frequency of dry conditions due to low rainfall.
A key factor in the water balance is evapotranspiration, which is the combined ‘use’ or loss of soil water by transpiration through plants and evaporative loss from the soil and other surfaces. Evapotranspiration is dependent on meteorological conditions – solar radiation, wind, temperatures, and humidity. It also depends on the ability of plants to extract and transpire water from the ground, which in turn is dependent on soil conditions.
Evapotranspiration is measured in millimetres of water depth, like rainfall. New Zealand pasture in average silt-loam soils, for example, needs about 5–6 mm of water per day in summer, and about 1 mm in winter, to meet both its physiological requirements and the atmospheric demand.
Often, this water requirement is not fully available, and the gap that develops between water demand and what is actually available is referred to as the potential evapotranspiration deficit or PED. This can be thought of as the amount of water that would need to be added, by rainfall or irrigation, to keep the pasture growing at its potential seasonal rate.
Days of potential evaportranspiration deficit
Days when water demand is not met, and pasture growth is reduced, are often referred to as days of potential evapotranspiration deficit. As a rule of thumb, an accumulation of 30 mm more PED corresponds to an extra week of reduced grass growth.
Many east coast regions of New Zealand, for example, typically experience about 400 mm of PED each year, resulting in about 3 months of reduced pasture production.
Droughts are hard to define because they arise from several causes, and can then affect different activities in different ways. PED is a useful means of ranking the severity of dry periods from a meteorological viewpoint. The choice of whether a dry period takes on the more sinister term of ‘drought’ might then be made on how often a given level of dryness (as defined by PED) might occur or be exceeded.
The well-remembered El Niño drought of 1997–98, for example, shown in the figure below, recorded 835 mm of PED in Marlborough, or more than 6 months of low or nil growth. The historical record in Marlborough shows that this level of drought occurs on average just once in 60 years. Other severely affected areas during this drought were Hawke’s Bay, Kaikoura, north Canterbury, and inland Otago.
The more recent 2000–01 drought recorded 802 mm of deficit in Marlborough. A drought like this occurs on average about once in 20 years. The difference in PED, 33 mm, represents a loss of about a week of pasture production.
Using historical patterns of climate variability and future scenarios of climate change, NIWA scientists have calculated that, for drought prone regions of New Zealand, the risk of drought is likely to increase during this century. The results of this study can be found at http://www.climatechange.govt.nz
For more information on water balance analysis for New Zealand, contact email@example.com
Wairarapa pastures in late autumn. The typical autumn recovery from low summer soil moisture may become less reliable under climate change scenarios projected for late this century. (Cover photo: Alan Blacklock)