4 Ecological character of Keta and Songor lagoons 4.1 Meteorology and hydrology 4.1.1 Climatic conditions The climate of the study area lies within the dry Equatorial climatic region of Ghana, which also covers the entire coastal belt of the country. This region is the driest in the country and is referred to as the central and southeastern coastal plains. The coastal lands of Ghana have two clearly defined seasons, the Dry season and the Rainy season. The Rainy season exhibits double maxima, the main one occurring between April and June and the minor one between September and October. June is normally the wettest month in the area. The annual isohyetal pattern of the coastal belt has the minimum in the west outside Accra up and close to Songor lagoon in the east. The prevailing wind direction is from the southwest (the southwest monsoons). This is a characteristic feature for the entire coastal belt of the country. Mean monthly averages of daily wind speed range between 21.1 to 29.0 km h-1. However, high velocity winds (110 km h-1) of short duration have been recorded in Accra. The north east trade winds rarely reach the coast. Day light and sunshine (hrs) in project area The day length varies between 11.8 h and 12.5 h in the study area. It reaches its maximum in June and minimum in January. Daily sunshine duration is least in June (4.8 h) when there is maximum cloud cover and maximum in November (8.4 h) with a mean of 6.9 h. The values in table 9 give an idea of the general variation in the hours of sunshine within the study area. Table 9 Day length (hours) and hours of sunshine within the study area Day length (hours) Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Mean 11.8 11.9 12.1 12.3 12.4 12.5 12.4 12.3 12.2 12.0 11.9 11.8 12.1 Hours of sunshine Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Mean 7.1 7.2 7.2 7.0 6.6 4.8 5.4 6.3 6.7 7.8 8.4 7.8 6.9 Relative humidity Relative humidity data for the study area are estimated using data at Ada and Keta. Since local variation in relative humidity is not appreciable especially within the same climatic belt, humidity values at Ada and Keta are considered representative of relative humidity for the Keta and Songor Ramsar sites. Generally, relative humidity is high in the mornings and at night, but is at a minimum in the afternoon (table 10). Temperature Long-term temperature records are available at the Ada synoptic station. Records at this station give minimum average temperatures between 23ºC and 26ºC whereas the maximum lies between 27ºC and 32ºC. August is normally the coldest month in the area. Records from the Keta observation station indicate that the minimum average temperature is 24ºC, whereas the maximum average is 31ºC. 29 Table 10 Percent relative humidity at Ada (5-year average) and Keta (14-year average) Ada Time Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Mean 00:00 89 90 90 88 89 91 94 96 94 91 89 91 81 06:00 90 92 89 91 93 93 95 97 95 93 92 92 93 12:00 71 74 74 76 77 82 81 80 78 75 74 71 76 18:00 83 86 85 84 85 88 90 91 90 88 86 87 87 Keta Time Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Mean 00:90 81 79 77 76 79 82 81 80 78 77 78 78 79 15:00 66 64 63 66 70 75 73 70 69 69 67 66 68 Rainfall The study area experiences two rainfall maxima with the annual average for different periods ranging from 688 to 855 mm (table 11). Rainfall occurs between March/April to July and September–October. The low rainfall gives rise to stream flow mainly in the Rainy season only. Between November and April, the numerous small streams that drain the area dry up; over the last decade even the Todzie River dried into a series of pools in its lower reaches. Rainfall in the greater Keta basin area was estimated using available records at Keta, Anyanui, Sogakope, Dabala, Anyako, Atiavi, Anloga, Adina, and Afiadenyigba. Rainfall records in the study area were reliable up to the 1980s. However, after this period, there are a lot of gaps in the data; only the stations at Keta and Ada have consistent data. Wakuti (1968) carried out an extensive analysis of rainfall in the study area and concluded that the variation in annual totals was small. Hence, for the southern section where the annual rainfall was about 900 mm, isohyets of the average monthly rainfall were not necessary. The arithmetic mean was considered adequate. In the present estimate, therefore, the arithmetic mean of the monthly rainfall is computed to give the mean basin rainfall for the Keta basin. This is presented in table 11 and illustrated in figure 5. Because Keta station has a long record, the variation in rainfall over various time frames was examined and this is also presented in table 11. It is clear from this presentation that between the mid-1970s and the early 1990s the mean annual rainfall has been low. Therefore the upper range of annual rainfall of 910 mm is on the high side. Long records of rainfall data are available for the Ada synoptic station. From the data, the following pattern of rainfall is observed. The maximum rainfall occurs in June with the major season itself beginning from March/April. There is also a minor season between September and October. The mean monthly variation is depicted in figure 5. A summary of the rainfall statistics for various time periods is presented in table 11. Whereas the long-term mean annual rainfall is 891.6 mm, the mean between the mid-1970s and the early 1990s falls below this long-term mean by over 23%. Pan evaporation Evaporation from the Keta and Songor areas was estimated using direct measurements of pan evaporation from Ada and Tema. It is recognised that both stations lie in the same climatic belt. It was observed that there is a decrease in evaporation from Tema to Ada. This difference notwithstanding, Ada and Tema records were close enough to be averaged to give the evaporation for the study area. The mean pan evaporation for the study area is presented in table 12 and illustrated in figures 5 and 6. 30 Table 11 Rainfall (mm) in the general study area and long-term rainfall in the Keta Ramsar site General study area Station Keta Anyanui Sogakope Dabala Anyako Ataivi Anloga Adina Afiadenyigba Mean STD Period 1913–1992 1955–1960 1953–1975 1954–1980 1955–1981 1957–1985 1957–1981 1956–1968, 1956–1981 1973–1981 Jan 10.6 12.2 21.8 14.2 10.4 6.4 12.0 8.7 5.1 11.3 4.6 Feb 22.6 1.4 21.3 18.1 28.9 22.3 22.5 28.3 34.2 22.2 8.7 Mar 56.1 116.8 87.2 68.7 71.0 61.7 50.7 73.4 65.6 72.3 18.6 Apr 99.2 173.1 111.4 103.8 105.3 97.3 124.8 130.4 118.3 118.2 22.2 May 155.2 227.3 136.4 178.7 129.0 140.6 156.7 216.2 130.1 163.4 34.6 Jun 187.5 228.2 211.4 194.2 222.1 246.4 306.4 284.2 248.8 236.6 37.3 Jul 64.7 93.4 51.6 52.1 61.4 81.3 66.1 68.2 49.8 65.4 13.6 Aug 19.9 32.2 22.2 25.6 26.2 17.7 14.5 66.0 17.5 26.9 14.7 Sep 49.8 16.5 57.9 43.8 49.2 45.0 40.8 54.9 25.1 42.5 12.8 Oct 88.3 143.2 102.2 108.3 78.7 87.6 88.0 128.9 74.7 100.0 21.9 Nov 34.1 29.4 61.5 68.1 29.4 35.7 32.6 41.6 30.5 40.3 13.7 31 Dec 12.8 13.9 16.4 8.9 10.8 7.7 7.8 12.0 8.5 11.0 2.8 Total 800.8 1087.5 901.3 884.4 822.3 849.5 923.0 1112.9 808.0 910.0 Mar–Jun 498.0 745.5 546.5 545.4 527.4 546.0 638.6 704.2 562.7 590.5 % 0.6 0.7 0.6 0.6 0.6 0.6 0.7 0.6 0.7 0.6 Sep–Nov 192.2 221.2 243.7 245.7 183.5 185.9 175.9 291.5 147.8 209.7 % 0.2 0.2 0.3 0.3 0.2 0.2 0.2 0.3 0.2 0.2 Keta Ramsar site (mm) Period Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Total 1913–1991 Mean 10.8 23.2 57.5 103.0 157.2 189.9 67.2 21.6 51.7 89.5 35.5 13.6 800.8 STD 19.8 32.7 48.1 57.3 79.1 134.6 84.5 45.7 64.0 69.3 34.0 21.8 CV 1.8 1.4 0.8 0.6 0.5 0.7 1.3 2.2 1.2 0.8 1.0 1.7 1915–1945 11.9 24.6 56.4 106.0 173.4 166.6 70.4 20.5 52.2 105.7 48.4 18.8 855.0 1946–1975 12.6 21.4 56.8 113.6 154.6 261.0 56.3 22.9 45.0 89.6 26.5 7.7 825.8 1976–1991 6.4 16.4 58.6 79.1 146.0 136.9 83.3 23.3 64.1 62.8 20.9 6.2 687.5 Table 12 Pan evaporation (mm) in the study area Station Period of Record Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Total Tema 1972–1980 146.9 162.1 194.1 183.7 173.6 143.3 150.2 133.6 148.0 172.2 176.0 148.8 1932.5 Ada 1961–1967 125.0 135.0 175.0 180.0 160.0 125.0 130.0 140.0 150.0 180.0 155.0 130.0 1785.0 Mean 135.9 148.5 184.6 181.8 166.8 134.2 140.1 136.8 149.0 176.1 165.5 139.4 1858.7 32 Ada Rain(mm) 9.2 21.2 65.7 110.5 170.7 235.6 63.3 19.3 53.2 85.3 42.4 15.1 (1915 to 1991) 33 Figure 5 Distribution of rainfall and evaporation at Keta Ramsar site 34 Figure 6 Distribution of rainfall and evaporation at Songor Ramsar site 4.1.2 Hydrological conditions Keta catchment In general, stream flow in the area is seasonal, and corresponds to the seasonal variation in rainfall.