202
D. O. Olago et al.
Thus, of immediate concern are the changes in nutrient dynamics and mixing regimes
within the lakes that are a major fishery resource (Victoria, Turkana, and Baringo);
these are in turn a result of increased thermal stability as they affect fishery production,
as has been observed in Lakes Malawi and Tanganyika, and can completely alter the
trophic structure of the food chain (O’Reilly et al. 2003; Verburg et al. 2003; ENSO
Project 2003). Such concerns extend to the inflowing rivers and wetland buffers along
the shores of the lakes.
2.2 Climatic Factors
The temporal distribution of precipitation over East Africa reveals daily to seasonal
and inter-annual variability, and a simultaneous decline in the number of wet days in
a year and increased frequency of intense rainfall (Kuya 2016). Rainfall in eastern
Africa shows robust teleconnections to the rest of Africa and to the global tropics. It is
strongly quasi-periodic, with a dominant timescale of variability of five to six years,
influenced in part by the El-Niño-Southern Oscillation (ENSO) phenomenon and
Sea-Surface Temperatures (SSTs) fluctuations in the equatorial Indian and Atlantic
oceans. Further, rainfall in the region tends to be enhanced during ENSO years
(Ropelewski and Halpert 1987; Ogallo 1989). An increase in the frequency of drought
has been reported in the semi-arid Lake Turkana region (Waktola 1999; Hann et al.
2003) and more recently also in many other parts of the country, most notably in
2010/2011 and 2015/2016, strongly linked to La Niña, with the milder droughts
attributed to low rainfall in typically arid to semi-arid areas that have a wide range
of year-to-year rainfall variability (Uhe et al. 2018).
The water balance of Lake Victoria is dominated by rainfall on the lake and
evaporation, with river inflow and outflow making minor contributions (Spigel and
Coulter 1996). In contrast, in smaller lakes, river inflows, and evaporation are the
dominant factors (e.g., Street-Perrott 1982; Finch and Hall 2006). As a consequence
of climate variability, it has been observed that lake surface areas and levels fluctuate
significantly, particularly for the smaller lakes. In addition, the very high potential evaporation which occurs throughout the year, which is in excess of 2000 mm
per year over large tracts of Africa, significantly affects water resources (Schulz
2001). Thus, relatively small changes in rainfall and evaporation may lead to shifting
between closed- and open-basin status, as was the case in historic times for some
of the lakes in Kenya and eastern Africa as a whole (Beadle 1981; Richardson and
Dussinger 1986; Verschuren et al. 2000; Bootsma and Hecky 1993; Owen et al.
1990; Spigel and Coulter 1996; Olago et al. 2007). For example, the surface area
of Lake Nakuru (mean depth 2.5 m) normally fluctuates between approximately
40 and 60 km
2 ; this translates into significant water level fluctuations. Sometimes,
the lake dries out almost completely, as happened between 1995 and 1997 (WWF
1998; McCord et al. 2009). On the other extreme, during prolonged sequences of
wet years, the water level rises to form a lake with a surface area of 50–60 km
2
(Onywere et al. 2013). Extreme events such as the El Niño (1982–3 and the 1997–8)
D. O. Olago et al.
Thus, of immediate concern are the changes in nutrient dynamics and mixing regimes
within the lakes that are a major fishery resource (Victoria, Turkana, and Baringo);
these are in turn a result of increased thermal stability as they affect fishery production,
as has been observed in Lakes Malawi and Tanganyika, and can completely alter the
trophic structure of the food chain (O’Reilly et al. 2003; Verburg et al. 2003; ENSO
Project 2003). Such concerns extend to the inflowing rivers and wetland buffers along
the shores of the lakes.
2.2 Climatic Factors
The temporal distribution of precipitation over East Africa reveals daily to seasonal
and inter-annual variability, and a simultaneous decline in the number of wet days in
a year and increased frequency of intense rainfall (Kuya 2016). Rainfall in eastern
Africa shows robust teleconnections to the rest of Africa and to the global tropics. It is
strongly quasi-periodic, with a dominant timescale of variability of five to six years,
influenced in part by the El-Niño-Southern Oscillation (ENSO) phenomenon and
Sea-Surface Temperatures (SSTs) fluctuations in the equatorial Indian and Atlantic
oceans. Further, rainfall in the region tends to be enhanced during ENSO years
(Ropelewski and Halpert 1987; Ogallo 1989). An increase in the frequency of drought
has been reported in the semi-arid Lake Turkana region (Waktola 1999; Hann et al.
2003) and more recently also in many other parts of the country, most notably in
2010/2011 and 2015/2016, strongly linked to La Niña, with the milder droughts
attributed to low rainfall in typically arid to semi-arid areas that have a wide range
of year-to-year rainfall variability (Uhe et al. 2018).
The water balance of Lake Victoria is dominated by rainfall on the lake and
evaporation, with river inflow and outflow making minor contributions (Spigel and
Coulter 1996). In contrast, in smaller lakes, river inflows, and evaporation are the
dominant factors (e.g., Street-Perrott 1982; Finch and Hall 2006). As a consequence
of climate variability, it has been observed that lake surface areas and levels fluctuate
significantly, particularly for the smaller lakes. In addition, the very high potential evaporation which occurs throughout the year, which is in excess of 2000 mm
per year over large tracts of Africa, significantly affects water resources (Schulz
2001). Thus, relatively small changes in rainfall and evaporation may lead to shifting
between closed- and open-basin status, as was the case in historic times for some
of the lakes in Kenya and eastern Africa as a whole (Beadle 1981; Richardson and
Dussinger 1986; Verschuren et al. 2000; Bootsma and Hecky 1993; Owen et al.
1990; Spigel and Coulter 1996; Olago et al. 2007). For example, the surface area
of Lake Nakuru (mean depth 2.5 m) normally fluctuates between approximately
40 and 60 km
2 ; this translates into significant water level fluctuations. Sometimes,
the lake dries out almost completely, as happened between 1995 and 1997 (WWF
1998; McCord et al. 2009). On the other extreme, during prolonged sequences of
wet years, the water level rises to form a lake with a surface area of 50–60 km
2
(Onywere et al. 2013). Extreme events such as the El Niño (1982–3 and the 1997–8)