Droughts

Periods of zero or low rainfall over a catchment area are easily observed, but the significance of a lack of rainfall, and what constitutes a 'drought', will vary in the different climatic regimes in the world. In some of the arid zones, for example, there may be several years in which no measurable precipitation occurs and the flora and fauna are adapted to these (normal) conditions. The shortage of rainfall in normally humid parts of the world can, however, result in serious water deficiencies, as for example in a failure of the monsoon in parts of India. Thus drought has widely different connotations according to location and consequences.

A meteorological drought is usually considered to be a period in which the rainfall consistently falls short of the climatically expected amount, such that the natural vegetation does not flourish and agricultural crops fail. Since such extremes are rarely experienced in the UK, low rainfalls more seriously affect water supplies for industry and domestic purposes (e.g. Murray, 1977; March, 2004). In the UK, with regular rainfall all the year round, the occasions of shortages were strictly defined in quantitative terms by the former British Rainfall Organization as an absolute drought where a period of at least 15 consecutive days occurs, none of which has recorded as much as 0.01 in (0.25 mm) of rain, and a partial drought when there is a period of at least 29 days during which the average daily rainfall does not exceed 0.01 in (0.25 mm);

(a) Water year 2001/2002

(a) Water year 2001/2002

Date

(b) Water year 1995/1996

Date

(b) Water year 1995/1996

Date

Fig. 9.21 Cumulative departures from mean daily rainfalls at the Hazelrigg site: (a) an average water year 2001/2002 with total rainfall 1080 mm; (b) a very dry water year 1995/1996 with average rainfall with total rainfall 727 mm. Note the difference in deficit scale.

Date

Fig. 9.21 Cumulative departures from mean daily rainfalls at the Hazelrigg site: (a) an average water year 2001/2002 with total rainfall 1080 mm; (b) a very dry water year 1995/1996 with average rainfall with total rainfall 727 mm. Note the difference in deficit scale.

but these criteria are no longer used in UK Met Office publications. Other forms of indexing for the severity of drought have also been used, such as the Herbst method used by Shaw (1979; and earlier editions of Hydrology in Practice) and the indices used in the Catalogue of European Droughts.

A good way of visualising rainfall deficiencies is shown in Fig. 9.21 in the form of accumulated departures from mean daily rainfall over a year. A very dry year (WY 1995/1996) with an overall deficit of 400 mm is compared with an average year (WY 2000/2001). This type of diagram was published in the publication British Rainfall for selected stations.

The evaluation of water resources requires the appraisal of the incidence of rainfall quantities over periods longer than daily sequences in order to determine storage capacities to meet demands. Hence more importance is normally attached to the identification of rainfall deficiencies in the long term. It has been a statutory requirement since 2003 in the UK that water supply companies produce drought contingency plans. Some droughts, of course, will be more severe than others (e.g. Table 9.7). Thus, the frequency characteristics of droughts of different durations are of interest in a similar way to extreme rainfall amounts. The maximum annual deficiencies can be analysed

Table 9.7 Minimum February-October rainfall totals for the UK (data taken from Marsh, 2004)

Rank

Year

Rainfall

% of 61-90

(mm)

mean

1

1921

520

70.1

2

2003

536

72.2

3

1959

557

75.0

4

1955

576

77.6

5

1929

598

80.6

6

1975

604

81.4

7

1911

606

81.6

8

1972

614

82.7

9

1919

620

83.5

10

1902

629

84.7

in a similar way to maximum annual peaks, but it is generally the case that there is greater interest in the frequencies of deficiencies in river flows than of rainfalls (see Chapter 11). Where there is not a flow-gauging site, information about rainfalls can help water engineers assess the potential safe yield of upland catchment areas; a typical assumption is that about 80 per cent of normal annual rainfall will arrive over the driest three consecutive years.

0 0

Post a comment