Mean Annual Precipitation, MAP: Concepts

The MAP (mm) characterises the long term quantity of water available to a region for hydrological and agricultural purposes. Under non-irrigated conditions it gives an upper limit to a region’s sustainable agricultural potential in regard to biomass production if other factors (e.g. light, temperature, topography, soils) are not limiting.

Not only is MAP important as a general statistic in its own right, but it is probably also that climatic variable best known to hydrologists and farmers, and to which they can relate many other things. In southern African agrohydrological studies MAP has, for example, been used as a variable related to monthly rainfall distribution, design flood prediction, the number of raindays or crop production (e.g. Schulze, 1983; Schmidt and Schulze, 1987; Dent, Lynch and Schulze, 1989).

While simple to calculate and attractive to use, the concept of MAP nevertheless has its weaknesses, in that in southern Africa

• negative departures of annual precipitation (i.e. low rainfall years) are more numerous than positive ones (i.e. higher than average years), and therefore annual rainfalls are not distributed normally (i.e. they have a positive skew),
• and MAPs are frequently inflated by a few very high annual totals from very wet years, especially in areas of low rainfall.

Distribution of MAP

The overall feature of the distribution of MAP over southern Africa is that it decreases fairly uniformly westwards from the escarpment across the interior plateau. Between the escarpment and the ocean in both the southern and the eastern coastal margins there is the expected complexity of rainfall patterns induced by irregularities of terrain. About 35% of southern Africa receives less than 300 mm per annum as a result of the presence of subtropical high pressure cells which inhibit rainfall generation because of predominantly subsiding air, while only about 7% has a MAP exceeding 800 mm. Perusal of the statistics indicates that KwaZulu-Natal is the wettest province, while the Western Cape has the highest variability of MAP within any of the provinces, and the highest individual point rainfall at an estimated 3345 mm per annum.

Annual Precipitation in the “Wettest” and “Driest” Years in Five

Using a frequency distribution of annual precipitation values, the “wettest year in 5” was represented by 20th percentile of exceedence and the “driest year in 5” by the 80th percentile. There is a general shift of isohyets eastwards for the driest year in 5 and westwards for the wettest year in 5.

Coefficient of Variation of Annual Precipitation: Concepts

The average amount of precipitation need not necessarily be a constraint to successfully carrying out an agricultural or water resources operation – in fact one can get by with, and adapt ones practices and operating rules to, a low rainfall if one has the assurance that the rains will fall when needed or as expected. Average annual precipitation maps, however, do not show the natural year-to-year variability of rainfall that occurs. For this reason the so-called Coefficient of Variation, or CV, expressed as a percentage, was mapped. The higher the CV, the more variable the year-to-year (i.e. inter-annual) rainfall of a locality is. Because this statistic considers deviations from averages by taking cognisance of whether the region has a high or low rainfall, it can be used for relative comparisons of variability between one region and the next.  The CV of annual precipitation is an index of climatic risk, indicating a likelihood of fluctuations in reservoir storage or crop yield from year to year. Agriculturally it is, perhaps, a more crucial statistic in marginal areas than in either very dry areas, where farming practices have adapted to variability, or in wet areas, where relatively lower inter-annual variabilities are generally expected.

A rule of thumb established over 50 years ago already by Conrad (1941), from analyses of rainfalls worldwide, is that the higher the MAP the lower its inter-annual variability was likely to be. In other words, areas with a low annual rainfall are likely to be doubly worse off, because they will additionally suffer from high deviations around their already low average rainfall. This inverse relationship between CV and MAP has been documented in several previous studies of rainfall over South Africa (e.g. Schulze, 1979; 1983).

Distribution of CV of Annual Precipitation The map of CV of year-to-year rainfall illustrates a general westward increase in the coefficient from less than 20% in the higher rainfall areas mainly along the eastern seaboard, to over 40% in the Northern Cape. It is important to note that on a monthly or seasonal basis, rainfall variability is considerably higher than on an annual basis (e.g. Schultze,1983).

Rainfall Concentration: Concepts

The distribution and selection of crops or the sustainability of veld or the period in which most runoff is generated, depend not only on annual amounts, variability or seasonality, but also on the duration of the rainy season, i.e. whether the rainfall season is concentrated over a short period of the year only or spread over a longer period.

A rainfall concentration index is calculated by a set of relatively complex equations developed by Markham. In essence a concentration index of 100% would imply that all of a location’s rainfall falls very concentratedly in one single month, whereas a concentration index of 0% would mean the rainfall of each month of the year is the same. The higher the concentration index, therefore, the less spread the rainfall season is over time (irrespective of whether it is a high or low rainfall area, or in a winter or summer rainfall region).

Distribution of Rainfall Concentration

The accompanying map of rainfall concentration shows that the highest concentrations, with percentages exceeding 65%, are in the north of the Northern Province and the Northern Cape, while the lowest rainfall concentrations are found along the south coast of the Eastern and Western Cape provinces, where values are below 10%, indicating an even spread of rainfall throughout the year. The tabulated statistics of rainfall concentration, derived from the 437 ooor gridded values covering southern Africa, confirm the Northern, North-West and Northern Cape provinces’ to having high concentrations, but additionally show that in those provinces the ranges of high concentrations between the lowest and the highest grid value are quite small. The Western and Eastern Cape with their low averages of the rainfall concentration, however, have a wide range between maximum and minimum values and consequently also display the highest coefficients of variation.

Rainfall Seasonality: Concepts

An area may have high or low average rainfall, a high or relatively low variability of rainfall from one year to the next or its rainfall may be concentrated over a short rainy season or spread over a longer period. That does not yet, however, indicate anything about the season in which the rain falls – be it predominantly in winter, or throughout the year, or in summer; and if in summer whether it be in early, late or very late summer. Rainfall seasonality is an important agrohydrological consideration, particularly when viewed in the light of runoff generation or, in agriculture, factors such as heat units.

Delineation of Rainfall Seasonality Regions

As with rainfall concentration, the seasonality of rainfall is best determined by the socalled Markham technique. To delineate southern Africa into regions of rainfall seasonality, the all yearrainfall region was first identified, by analysing the median (i.e. expected) monthly rainfalls at each of the 437 000 grid points over the region. All grid points with a rainfall concentration < 20% were designated all year rainfall. Next, the winter rainfall regions were isolated by a Smoothed Percentages index. These were grid points where the smoothed percentage index of the annual rainfall for each of the months June, July and August exceeded 8% (i.e. P%6 P%7 P%8 > 8). The greater part of South Africa is, of course, in the summer rainfall zone and the smoothed percentages index was again applied, this time to subdivide the remaining parts of the country into early summer (with rainfall concentration in December or earlier), mid-summer (peak concentration in January), late summer (February) and very late summer (with peak concentration March to May) zones.

Why Monthly Rainfall Totals?

There are many water resources as well as agricultural problems and decisions for which MAPs or even wet/dry season precipitation totals, be they high or low, are of relatively little consequence, because an intra-year distribution of rainfall is required. Monthly rainfall values serve as an important tool in describing such an intra-year distribution. It should, however, be borne in mind that the use of the calendar month is but a time step of convenience for describing temporal patterns of rainfall, for it breaks up annual precipitation into components of time long enough to smooth out many of the irregularities of daily rainfalls. Nevertheless, large differences can exist from one month to the next. Some of these differences result from major rainfall generating mechanisms changing from one month to the next, for example, when general and gentle frontal rains of winter and early summer are replaced with more localised convective storms which characterise the mid-to late summer months. Monthly rainfall is a major contributing factor to the calendar of agrohydrological activities and the month is used, for example, to define the start, duration and end of the rainy season, and also the beginning of the hydrological year (October).

Why Median and not Mean Monthly Rainfall?

The median is the middle value when a data series is ranked from highest to lowest. It therefore designates a statistically expected value, with as many years having been wetter than the median as there are years having had less rain than the median value. Mean values are frequently inflated by a few heavy and extreme events, or outlier events, that may have occurred – a phenomenon which is especially prevalent in dry regions or in generally dry months. Under such rainfall regimes the mean is therefore not as representative of expected conditions as the median.

The Importance of Rainfall to Agriculture

Among the various individual climatic parameters which influence the growth characteristics of crops in southern Africa the most important is considered to be water. Limitations in water availability are frequently a restrictive factor in plant development, and water is essential for the maintenance of physiological and chemical processes within the plant, acting as an energy exchanger and carrier of nutrient food supply in solution. In any regional study of agricultural production rainfall is therefore of fundamental importance. Focus is invariably on the patterns of rainfall in time and over an area, by asking initially

·         how much it rains where it rains (its spatial distribution)

·         when it rains (its seasonal distribution)

·         how frequently it rains and

·         what the duration and intensity of rainfall events are.

In their analyses of rainfall, however, the concerns of farmers go further, since they need to consider also how variable the rainfall is from year to year or for a given month and how frequently droughts of a certain level of severity are likely to recur. The reservoir of water from which crops draw their moisture supply through the soil is derived mainly in the form of rainfall, with relatively minor contributions in southern Africa from dew, fog and snow. Not all rainfall is, however, freely available to the crop through the soil, as some is intercepted by the plant before reaching the soil, part runs into streams as stormflow after rainfall events (without being utilised by plants), some percolates into the deeper soil layers beyond the root zones and a portion is evaporated directly from the soil surface without being transpired through the plant.

Distribution of Median Monthly Rainfall, February-May

The distinctive feature of these four months is the transition from areas with summer rainfalls to areas with winter rainfalls. This is evident particularly in April with its similar rainfall patterns and inter-quintile ranges (i.e. the difference between the 20% and 80% exceedence probability values) throughout southern Africa. Up to April, KwaZulu-Natal remains the wet province while the Western Cape is the dry province until the end of March.

Distribution of Median Monthly Rainfall, June-September

June to August are very much the months in which the Western Cape is the only province to really feature in terms of median rainfall amounts. The summer rainfall regions, with the virtual absence of rainfall except along the east coast and where it is orographically induced by mountains, is characterised by CVs often being meaninglessly high (designated with a * in the tables) because they have been computed from low base values, with a small number of outliers distorting the statistic. September patterns show the gradual phasing in of summer rains.

Distribution of Median Monthly Rainfall, October-December

Gauteng, Mpumalanga and KwaZulu-Natal feature as the generally wet provinces in these months, with the decline of precipitation particularly obvious in the winter rainfall areas of the Western Cape, which also displays high intra-provincial CVs in the last three months of the year.