In terms of the physical effectiveness of water harvesting, this has been established beyond a doubt both by the use in semi-arid and arid areas around the world for at least 10,000 years, as will be seen from the review below, and by modem studies o f soil water balance and yield improvements vis-à-vis in situ rainfall receiving controls. Water harvesting has always received much greater attention from practitioners than researchers, and as the ‘twain often n’er will meet’, the amount of documented research into water harvesting is remarkably small compared to both its historical importance and its current potential. One suspects that this disparity is Hkely due to: the lack o f famiharity with the technique by researchers from temperate climates, who are more familiar with practices of in situ water conservation; the relatively large areas which would be require on-station for controlled experiments, with high costs of fencing and maintenance etc; and the research funding and publication incentive stmcture, which rewards process-based studies which in turn require high degrees of experimental control on a narrowly defined question at a fine scale. Until relatively recently, as well, there has been strong research and development funding in developing countries for irrigation and fertilisation, which has caused water harvesting to be overlooked.
(t r a n)
Ifunôrfj
SRUN: "4-R U N O N AREA > •; RUNOFF AREA D ire ctio n ^ E a r th B u n d • / Crist o( bunaw . J h ig h ir th a n jn tr o tio n f i M i t t l i p o f Areo c o n c e n tr o tio nFigure 2.1 Water harvesting systems (counterclockwise from upper left); a) Idealised system (Lovenstein 1991) DPER = deep percolation b) Macro and micro systems (Tauer and Humborg, 1992) c) Tied contour bunds used in World Bank project in Baringo (Critchley, 1987) d) Lunettes or crescents used in F AO project in Baringo (BPSAAP 1984).
m a c r o c a t c h m e n t s y s t e m s runoff area slope > 1 0 % m i c r o c a t c h m e n t s y s t e m s slope < 1 0 % cropping area runoff areas cropping areas C u to tt D rain I sometimes necessary Contour Ridges ot 1 5m oport lopproK] wilti C ross-ties MCTMW « ,
The US Office of Technology Assessment, in a systematic study of promising techniques for agriculture in the Sahel, found water harvesting to be the most likely to succeed as an introduced technology on criteria of cost, technical knowledge required (OTA 1988). A major international convocation of water harvesting experts took place in Cairo in 1993 (FAO 1994), involving academics and practitioners from 13 countries and five international agencies, indicating the widespread interest in the promise of water harvesting. On purely agronomic criteria, water harvesting is an unquahfied success, having repeatedly been shown to enhance soil moisture in the root zone over the growing season and as such to increase crop yield, particularly in dry years (Ben Asher 1988) when this biomass is most needed. It is important to note that the interval between (runoff producing) storms increases as the annual rainfall decreases, as can be seen from Figure 2.2.
From this figure it is apparent that for a decrease in annual rainfall from 700 mm to 300 mm results in an increase in dry spells, where a dry spell is defined as the interval between two storms of greater than 10 mm, from 11 to 18 days. Thus the lower the annual rainfall the more
advantageous, ceteris paribus, is water harvesting. In practice the lower the rainfall the more that grazing rather than cropping is to be the economic mainstay, and the former implies mobility and hence a disincentive to invest in creating structures at a particular locality. In addition, there are physical considerations regarding the minimum effective catchment area as rainfall declines; and the larger the catchment area the greater the velocity of the flows generally will be, leading to risks o f erosion of the runon area and the technical and economic requirements, as well as the level of social organisation required to control the runoff is substantially increased.
The study o f the effectiveness of water harvesting in terms of soil moisture has been most researched in the ‘cross over’ area between micro water harvesting and quasi in situ water conservation, in so far as the latter involves topographic modification so as to create a runoff contributing zone. Studies of ‘pure’ water harvesting, particularly the well known stone lines used successfully in Yatenga, Burkina (which were a successful adaptation of an Israeli technique (Atampugre 1993), and subsequently extended elsewhere in the Sahel, have also demonstrated the effectiveness of water harvesting from an agronomic perspective. In Figure 2.2, a it can be seen that in comparison with rainfed agriculture in Mali, the plot under water harvesting quadruples available soil water, which in turn is two thirds of the figure o f irrigated agriculture. The question of runoff efficiency, which is not really subject to human control at least at an economic cost, in spite o f many such trials in the U.S. in the 1970’s and Israel until present, is particularly germane
(from left)
a) Comparison of available water in root zone under three modes of water supply, Mali. Upper and lower bars represent saturation and wilting point. The extra soil water under water harvesting allows survival during dry spells in the rainy season (Klemm 1986).
b) The length of dry spells within the rainy season increase, as can be seen from this data from the Sahel, with decreasing annual rainfall; Baringo has a relatively high average rainfall of 630 mm, but with a bimodal distribution, and thus a lengthy dry spell
(Tauer and Humborg, 1992).
W ater in root z o n e (cm ) rainfed 150 160 170 180 100 2 00 210 220 2 3 0 2«0 250 260 270 2 60 200 3 0 0 W ater harvesting 150 too 170 180 190 200 210 220 2 30 240 250 260 270 2 8 0 2 0 0 3 0 0 irrigated Julian day
days
ETO in m m /d ay 60 50 40 30 20 + + 10 0 3 300 400 500 600 700 m ean an n u a l rainfall (1 9 7 4 -1 9 8 8 ) in mm8 m axim um dry period + potential évapo tran sp iratio n • n u m b er of rainy d a y s
to this present study, which is more focused on the assessment of suitability end of water harvesting than the structure design and optimisation modelling end of the water harvesting, as the latter has received far more attention than the former.