2. Drought Risk under Current Climate
Key Points:
- In order to quantify the likely effects of climate change on drought risk around the country, we must first provide quantitative estimates of the current drought risk.
- The driest parts of the country (Gisborne, Hawkes Bay, Marlborough, most of coastal Canterbury, and inland Otago) experience annual water deficits in the 300-500mm range. The average annual PED in coastal Marlborough can exceed 600mm.
- The incidence of drought varies from year to year. El Niño tends to bring drier conditions to the northeast of both the North Island and the South Island. La Niña can also bring drought to the eastern South Island.
2.1 Typical Levels of Potential Evapotranspiration Deficit
In this report we measure the incidence and severity of droughts in New Zealand in terms of the potential evapotranspiration deficit (PED). As we explained in section 1.2, PED was calculated daily on the 0.05° national grid, and accumulated for each calendar month over the 31-year period July 1972 to June 2003. Figure 2.1 (left panel) shows the total July to June PED averaged over the 31 years of gridded data. In the wettest regions of the West Coast and at high altitude, annual PED is close to zero, meaning that at no time of year is there a pasture deficit. The driest parts of the country (Gisborne, Hawkes Bay, Marlborough, most of coastal Canterbury, and inland Otago) experience annual deficits in the 300-500mm range (green shading), except for the coastal tip of Marlborough where calculations suggest the average annual PED could exceed 600mm.
Figure 2.1 Accumulated July-June PED (mm) calculated from 0.05° gridded data set: average over 31-year period 1972/73 to 2002/03 (left), and PED levels in extreme drought year of 1997/98 El Niño (right).
2.2 Year to Year Variability in Drought
Drought incidence will vary from year to year, and is often associated with El Niño-Southern Oscillation (ENSO) variations. ENSO is a natural fluctuation of the tropical Pacific atmosphere and ocean. In the El Niño phase, the easterly tropical trade winds weaken and tropical sea surface temperatures can be several degrees above normal. New Zealand often experiences stronger than normal southwesterly airflow, with lower temperatures across the country and drier conditions in the northeast of both Islands. The La Niña phase results in higher pressures and more settled weather over southern New Zealand, which can also bring drought to the eastern South Island.
Figure 2.1 (right panel) shows that PED exceeded 600 mm in a substantial part of eastern New Zealand in the severe drought year of 1997/98, which coincided with a strong El Niño in the tropical Pacific. Drought can affect different parts of the country in different years. Figure 2.2 suggests one way to get an integrated national picture of historical drought severity: time series of the percentage of gridpoints with PED exceeding some specified level immediately highlight the years of widespread drought. The big El Niño years of 1972/73, 1977/78 and 1997/98 stand out.
Clearly, any ranking of drought severity requires specifying a PED threshold. Drought impacts cannot be easily related to a uniform PED threshold across the country, since many eastern regions experience a PED of more than 200mm almost every year, and local farmers have adapted their operations to this. However, this same level of dryness might well impact adversely on farming operations in other, normally wet parts of the country (e.g. Manawatu). It can nonetheless be useful to plot the total area of the country that exceeds any set of thresholds (e.g. PED levels of 200, 400 and 600mm in Figure 2.2) as an indication of how widespread drought conditions were in any given year. For an annual PED accumulation of 400mm or more, there were 9 years out of the 31-year historical record, where more than 10% of the country was "in drought". These years, in decreasing order of severity, were: 1997/98, 1972/73, 1977/78, 2000/01, 1982/83 and 1988/89, 2002/03, 1981/82, and 1984/85.
Figure 2.2 Fraction of gridpoints (on 0.05° grid) with July-June PED accumulation exceeding specified threshold
2.3 Statistics of Current Drought Risk
There are various ways in which drought incidence can be characterised statistically. In this report we focus primarily on drought intensity (the level of PED that is reached in a given year and location) and drought risk (the probability that a given level of PED is exceeded in a given year at a given location). However, drought duration is also an important issue in dry-land farming. There is of course a fairly strong link between drought intensity and duration, with stronger droughts generally lasting longer.
We can also analyse annual drought risk in terms of its return period. The return period of an event is defined as the inverse of its exceedance probability: thus,
Return Period 
where the variable X is drought intensity (in mm PED), P indicates probability, and xT is the magnitude of a drought having a return period of T. For example, if there was a 5% chance (0.05 probability) of annual PED exceeding 600mm, then we would say that event (PED of more than 600 mm) has a return period of 20 years. Note that a 'one-in-twenty-year' or 'twenty-year average recurrence interval' event will not occur precisely at 20-year intervals. The numerical method we have used for estimating return periods is detailed in the Technical Appendix.
Figure 2.3 Probability that in any one year the annual accumulated PED will exceed 200 mm (left) and 600 mm (right)
2.3.1 Probabilities of Selected Levels of Drought Index
Figure 2.3 shows the probability that PED will be more than 200mm (left) and 600mm (right) over New Zealand. (A threshold of 400mm was also used - see Appendix 6.8). In the dry eastern regions, a PED of 200mm corresponds approximately to about 1.5-2 months of deficit, and 600mm to 5 months. The regions most susceptible to droughts appear as brown-red shading in the left box of Figure 2.3, where the probability of at least 200m PED deficit exceeds 0.5; ie, at least 1 year in every 2 we would expect a deficit of 200mm to be exceeded, and pasture to have insufficient moisture for optimum growth for up to 2 months over the summer period. From the right-hand panel of Figure 2.3, PED exceeding 600mm is a 1-in-20 year event at the boundary between the dark green and light green shading.
2.3.2 1-in-20 Year Return Period
In this report, we use a 1-in-20 year PED as a convenient measure of 'severe drought', even while recognising that in wetter parts of the country a 1-in-20 year event is not that dry in absolute terms. Rather than mapping probabilities (as in Fig 2.3), we could map return periods. Figure 2.4 shows the PED levels corresponding to a 1-in-20 year return period. This gives us a picture of just how extreme droughts can become under the current climate. The 1-in-20 year return period PEDs for the Lincoln and Napier gridpoints are 770mm and 748mm, respectively. The most extreme individual years in the 1972/73 to 2002/03 data recorded a PED deficit of 851mm (in 1988/89) at Lincoln, and 799mm (1997/98) at Napier. During these extreme years, pasture was in moisture deficit for 192 days at Lincoln and 174 days at Napier.
