INTER-ANNUAL AND SEASONAL DROUGHTS 4.1 Occurrence and Persistence

5)

spectra in both annual/seasonal rainfall. The highest contribution (60%) by this oscillation was found in Central parts of Tanzania.

3 - 5 years oscillation was found in all the regions in the annual as well as in the seasons.

Strong peaks are clear in northeastern areas. During Vuli and Masika combined spectra for this oscillation was found to account for 50% in all annual and seasonal rainfall.

9 - 12 year cycles was missing in the Unimodal (October - April rainfall) in all the regions but present in the annual as well as Masika and Vuli rainfall spectra.

19-year oscillation, which accounted for 58% of the spectra, was found to be strong in the extreme western parts of the country when annual rainfall was analyzed.

This oscillation is also found in the Masika and October - April rainfall where its contribution is not as large. Vuli rainfall does not reveal the presence of a 19-year cycle in all the regions considered.

The above mentioned oscillations have been found by Nyenzi (1988) and Ogallo (1980) among many others. The authors suggested that the two-year cycle is associated with Quasi-Biennial oscillation while the 3 - 5 years possibly related to fluctuations in the Southern Oscillation. Sun spot activity with a cycle of 10 years has been, actively pursued to be associated with the 9 - 12 year’s peak in rainfall. This peak is also found in the spectral analysis of Tanzania rainfall. Finally, the 19 years oscillation seems to be a multiple of the 10 years cycle and this could be associated with the repetitive alignment of sun, earth and moon.

4.0 INTER-ANNUAL AND SEASONAL DROUGHTS 4.1 Occurrence and Persistence

Drought as defined under section I:3 is used to examine the time series of area averaged rainfall over the country. Station data, which indicated probability of a drought in one out of 4 years (25%) or higher were used in the computation of area average rainfall and to delineate regions prone to droughts. Inter annual droughts were frequent in regions II, V, and VII. Seasonal droughts were frequent in all the seven regions. Inter annual droughts were found to be higher (8 in 32) years in region V while the lowest (5 in 32 years) occurred in regions IV, VI and VII.

shows the space distribution of inter annual drought frequencies probability over the country. It is evident that droughts have higher probability over central and northeastern areas.

This is the region where annual rainfall is below 800mm. Low values occur over West Lake Victoria and Western, southwestern, southern and coast regions.

Persistence ratios are shown in which Low values coincide with low values of probability and large values found in the same areas as for high probability. Almost all the ratios are negative indicating that inter-annual drought alternate.

The unimodal (Ott - Apr) region (III, V, VI and VII) seasonal rainfall indicated drought probability of less than 25% suggesting that droughts are infrequent in this region during the rainy season. The southern, southwestern and western parts of the country, the rains for these regions are very reliable as indicated by the low coefficient of variation in this season. Drought frequencies are generally below 6. Central Tanzania has a high frequency of droughts during this season (over 6).

Persistence ratios are small and negative in most areas.

Masika (March - May) rainfall indicates high seasonal droughts over northeastern parts of the country. Lowest values are found over Western Lake Victoria Basin. Drought frequencies are large over northern coast, central and east of Lake Victoria Basin. Likewise low values are indicated over western parts of Lake Victoria. Persistence ratios reveal large negative ratios over the country. This implies that there is a good chance of drought persistence in these areas during masika.

During Vuli season (Ott - Dee) season the probability of drought occurrence in northern Tanzania is about 30 - 40 percent. The northeastern area has the highest probability. shows the persistence ratios. It is seen from this figure that the negative ratios are high over northeastern parts of the country where values greater than 0.32 exist. Ogallo (1984) also observed the alternating character of these droughts.

Low negative persistence values prevailed in regions III and V while a high negative persistence value occurred in region II and parts of region IV. This high value is associated with the frequent inter-annual rainfall deficits that occur over the eastern area of lake Victoria (Kavishe, 1996). Auto correlation function analysis indicates persistence pattern in annual rainfall for most of the regions.

The 95% confidence limits were applied to find significant correlation.

4.2 Duration

Annual area averaged rainfall for the seven regions were examined during the period 1963 - 1994.

It was found that the longest duration is 5 years, which occurred in region VI between 1990 -1994 where the average inter-annual drought duration was found to be 2.2 years.

At the seasonal (October - April) level region III experienced the longest duration of a drought of 2 years from 1973/74 - 1974/75. Average seasonal drought duration is 1.3 years in this season.

Masika rainfall (March - May) show that the longest drought duration was 4 years (1982 - 1985)

186

which occurred in region II. Low average seasonal drought duration of 1.2 was indicated during the study period. Vuli (October - December) season registered a drought of 4 years that lasted from 1973 - 1976 in region II. This period has been noted as the time when the whole of East Africa was drought stricken (Ogallo 1984). Average seasonal drought duration was observed to vary from 1 .O

to 1.3 in the whole bimodal region. In general the average inter annual drought duration was found to be higher than the average seasonal drought duration except in regions II, III, and I where average seasonal drought duration is higher than the average inter annual duration.

gives the inter-annual probability of n-year long drought for region VI where the longest drought duration (5 years) was experienced. It can be seen that the probability of having a six consecutive years drought is 4.2% while for a seven years consecutive drought the corresponding value is 2.7% which gives a return period of 36 years.

Vuli season was found to have the longest drought duration (4 years) among all the rain seasons of the year. depicts the Vuli probability of n-year long drought for region II. We conclude from figure 42 that a 5 year drought probability is 5.7% while a six year consecutive drought has a probability of 3.6% which corresponds to return periods of 17 years and 27 years respectively.

4.3. Severity

Run-sums as defined by Yevjerivh (1967) in section 2.2.1 (vi) were used to study the severity of droughts in Tanzania. Mean run-sums were calculated for the drought years during the period of study. The highest annual run-sum means appear in table 11 for five regions.

From table 1 it is seen that interannual drought severity was pronounced in region VI during 1990 - 1994 where the run-sum mean was 49% of the annual mean rainfall. This was followed by region III in 1969 with a run-sum mean of 35% of mean annual rainfall. The lowest run-sum mean of 19%

is indicated for region IV in 1983 - 1986 period.

Table 1: Highest annual Run-Sums during droughts

Region Year Run length Run-Sum Total Mean

Years (mm) % of MAP % of

MAP

Region II (North Eastern areas)

Region III (Central areas) Region IV (Lake Victoria Basin)

Region V (western area) Region VI (South Western

1965 - 1966 2 559 55 27

1969 1 233 35 35

1983-1984 2 648 58 19

1975-1997 3 671 75 25

1990-1994 5 3123 245 49

Unimodal rainfall region experienced highest run-sum mean of 32% of mean seasonal (Ott - Apr) rainfall in region VI during the drought period of 1987/88.

Vuli (Ott-Dee) rainfall indicates that region IV and I experienced high run-sum means of 45% of mean seasonal rainfall during droughts of 1973 - 76 and 1980 - 8. Region II attained a run-sum mean of 40% of the seasonal mean precipitation.

Masika season (Mar - May) was characterized by droughts that reached a run-sum mean of 42% of seasonal mean over region IV and 29% in region I during the early 1970’s. Region II suffered a drought in 1993 - 1994 with a run-sum mean of 40%.

Table 2 shows the severe droughts that occurred during the study period. From this table we may safely conclude from the run-sum means that droughts are more severe in the bimodal region of Tanzania during the seasons.

Table 2: Severe Droughts

BIMODAL

AtUlUd Ott - Apr Mar - May Ott - Dee

Region I

(North coast) 1993 1971, 1974, 1976

188

---~- . - -.-- - __I-- .-. ...l_--_..~-._ _- .__. .--- ._ ._--

Region II (North Eastern areas)

Region IV (Lake Victoria Basin)

UNIMODAL

1966, 1975

1973 1981, 1993

Region III (Central areas)

Region V (Western areas)

Region VI 1987,1990,1991 (South Western

areas) Region VT1 (South Coast)