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Moisture conditions in the savanna region of West Africa
Agricultural Meteorology, 15(1975) 133--136 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands. Book Reviews
Moisture Conditions in the Savanna Region o f West Africa. Kala Swami. McGill University, Toronto, Ont., 1973, 106 pp., $8.50. The moisture conditions in the savannas of Africa are of great topical interest at the present time because of serious drought conditions that have brought widespread famine. Kala Swami has produced a clearly written and well-documented book comparing moisture conditions in the west African savanna to adjacent regions of forest and desert, from a climatic viewpoint. Although many of the concepts discussed and supported by this work have appeared numerous times in the literature, the clear and orderly presentation of a curious mix of annual, m o n t h l y and daily weather data provides valuable insights into the importance of relatively small-scale meteorological events, such as thunderstorms, as well as the larger-scale events long known to be important. It is my belief that a more meaningful marriage of synoptic meteorology and " m e a n annual" climatology, such as is suggested by Kala Swami, could only improve our understanding of world climates. The book includes a brief review, which is supported by a considerable number of references, of present ideas concerning the causes of savannas as well as a description of the physical characteristics and natural environment of the west African region under investigation. Since the object of the book is to compare moisture conditions (as a climatic factor) in a savanna to those in adjacent "non-savanna" regions, considerable effort is expended to obtain a region that is unequivocally savanna by the agreement of six different investigators using three criteria: physiognomic, taxonomic and ecological. Having carefully delineated the regions of savanna, desert and forest, the frontal region (termed surface disturbance) between the continental tropical air over the Sahara and the maritime tropical air over the Atlantic, is characterized in detail by partitioning it into four zones and estimating the position of each zone from years of daily surface synoptic maps. Although considerable judgement undoubtedly was involved in characterizing daily conditions, the frequency of occurrence of various zones clearly is coincident with savanna boundaries. One point that is emphasized is that one can not distinguish the savanna boundaries on the basis of mean annual rainfall alone; rather the annual distribution of rainfall is crucially important. Much of the book is devoted to the investigation of this annual distribution of precipitation as well as potential evapotranspiration, vapor pressure deficit and relative humidity. At this point I would like to comment on the thoughtful organization of the book. Chapter 4 deals with "Method and Level of Inquiry", and the reader is exposed to a synopsis of the upcoming ideas through a detailed
134 look at m o n t h l y data from 5 representative stations. The essential features of the annual moisture distribution are illustrated very clearly without the usual frustration of having to digest an overwhelming a m o u n t of data. The essential features of the savanna being a reliable water supply for 2 to 3 months along with a reliable dry period of 4 to 5 months. In contrast the rain forest has a less reliable water supply over a longer season, with greater total water accumulation and the absence of a dry season; the northern transition to desert is characterized by year around dryness. These concepts are amplified in the following three chapters with abundant data that is presented in a concise " m a t r i x " table form that is very efficient. The chapter on water balance is the weakest in the book since the role of the soil is excluded; although this is convenient, and in keeping with the stated goal of investigating climatic effects, one might hardly expect the savanna to respect such an arbitrary dissection of the relevant variables -- soil, plant and atmosphere. The last chapter, which deals with daily moisture conditions, is one of the most interesting even though the meteorological analysis is far from exhaustive. With hourly data from several stations the author demonstrates a strong similarity between the daily precipitation characteristics for savanna and forest areas during the wet seasons. However, using thunderstorm-path information from the literature, which coincides remarkably well with the savanna boundaries, along with vapor-pressure-deficit data, Kala Swami suggests that the higher evapotranspiration demand accompanying the small-scale thunderstorms in the savanna favors different vegetation characteristics than the low evapotranspiration demand associated with large-scale storms common to the tropical rain forest. I feel that this point deserves considerably more investigation than the somewhat weak vapor-pressure-deficit argument presented. However, this may be difficult if the necessary data is unavailable. Many subjects for further investigation are evident from this work and this is a sign of a good book. For example, there has been little attempt to address the " f e e d b a c k " effects between climate and vegetation and how the effect of a diffuse climatic gradient might be sharpened up by competition between various species of plants. A more timely topic, which this book could contribute significantly to, is the dependence of the surface disturbance location and movement on the general circulation; this, of course, will involve quantifying the dynamic character of the savanna and perhaps its stable or fragile nature. Kala Swami has produced a relatively short and detailed book with a wealth of information on climatic moisture conditions in the west African savanna. The material is so clearly presented that it can be digested, for the most part, in several hours. JOHN M. NORMAN {University Park, Pa.)
Moisture Conditions in the Savanna Region o f West Africa. Kala Swami. McGill University, Toronto, Ont., 1973, 106 pp., $8.50. The moisture conditions in the savannas of Africa are of great topical interest at the present time because of serious drought conditions that have brought widespread famine. Kala Swami has produced a clearly written and well-documented book comparing moisture conditions in the west African savanna to adjacent regions of forest and desert, from a climatic viewpoint. Although many of the concepts discussed and supported by this work have appeared numerous times in the literature, the clear and orderly presentation of a curious mix of annual, m o n t h l y and daily weather data provides valuable insights into the importance of relatively small-scale meteorological events, such as thunderstorms, as well as the larger-scale events long known to be important. It is my belief that a more meaningful marriage of synoptic meteorology and " m e a n annual" climatology, such as is suggested by Kala Swami, could only improve our understanding of world climates. The book includes a brief review, which is supported by a considerable number of references, of present ideas concerning the causes of savannas as well as a description of the physical characteristics and natural environment of the west African region under investigation. Since the object of the book is to compare moisture conditions (as a climatic factor) in a savanna to those in adjacent "non-savanna" regions, considerable effort is expended to obtain a region that is unequivocally savanna by the agreement of six different investigators using three criteria: physiognomic, taxonomic and ecological. Having carefully delineated the regions of savanna, desert and forest, the frontal region (termed surface disturbance) between the continental tropical air over the Sahara and the maritime tropical air over the Atlantic, is characterized in detail by partitioning it into four zones and estimating the position of each zone from years of daily surface synoptic maps. Although considerable judgement undoubtedly was involved in characterizing daily conditions, the frequency of occurrence of various zones clearly is coincident with savanna boundaries. One point that is emphasized is that one can not distinguish the savanna boundaries on the basis of mean annual rainfall alone; rather the annual distribution of rainfall is crucially important. Much of the book is devoted to the investigation of this annual distribution of precipitation as well as potential evapotranspiration, vapor pressure deficit and relative humidity. At this point I would like to comment on the thoughtful organization of the book. Chapter 4 deals with "Method and Level of Inquiry", and the reader is exposed to a synopsis of the upcoming ideas through a detailed
134 look at m o n t h l y data from 5 representative stations. The essential features of the annual moisture distribution are illustrated very clearly without the usual frustration of having to digest an overwhelming a m o u n t of data. The essential features of the savanna being a reliable water supply for 2 to 3 months along with a reliable dry period of 4 to 5 months. In contrast the rain forest has a less reliable water supply over a longer season, with greater total water accumulation and the absence of a dry season; the northern transition to desert is characterized by year around dryness. These concepts are amplified in the following three chapters with abundant data that is presented in a concise " m a t r i x " table form that is very efficient. The chapter on water balance is the weakest in the book since the role of the soil is excluded; although this is convenient, and in keeping with the stated goal of investigating climatic effects, one might hardly expect the savanna to respect such an arbitrary dissection of the relevant variables -- soil, plant and atmosphere. The last chapter, which deals with daily moisture conditions, is one of the most interesting even though the meteorological analysis is far from exhaustive. With hourly data from several stations the author demonstrates a strong similarity between the daily precipitation characteristics for savanna and forest areas during the wet seasons. However, using thunderstorm-path information from the literature, which coincides remarkably well with the savanna boundaries, along with vapor-pressure-deficit data, Kala Swami suggests that the higher evapotranspiration demand accompanying the small-scale thunderstorms in the savanna favors different vegetation characteristics than the low evapotranspiration demand associated with large-scale storms common to the tropical rain forest. I feel that this point deserves considerably more investigation than the somewhat weak vapor-pressure-deficit argument presented. However, this may be difficult if the necessary data is unavailable. Many subjects for further investigation are evident from this work and this is a sign of a good book. For example, there has been little attempt to address the " f e e d b a c k " effects between climate and vegetation and how the effect of a diffuse climatic gradient might be sharpened up by competition between various species of plants. A more timely topic, which this book could contribute significantly to, is the dependence of the surface disturbance location and movement on the general circulation; this, of course, will involve quantifying the dynamic character of the savanna and perhaps its stable or fragile nature. Kala Swami has produced a relatively short and detailed book with a wealth of information on climatic moisture conditions in the west African savanna. The material is so clearly presented that it can be digested, for the most part, in several hours. JOHN M. NORMAN {University Park, Pa.)