IMPORTANCIA DE LA IMPLEMENTACIÓN DEL PLAN DE ESTRATÉGIAS

In temperate environments, the infiltration rate into soils are generally considered to be controlled by the soil water capacity, which is essentially a function of soil texture and soil depth (Baver et al 1972). The rate of decrease in infiltration in soils with a good stmcture is a result primarily of the decrease in matric potential as the soil moves to a saturated state (Figure 3.1, a). The infiltration rate for a particular soil texture and depth, subjected to rainfall o f various intensities, results in various initial infiltration rates (= the rate o f rainfall application, up until the moment of ponding), but eventually all the infiltration curves will tend towards one, final, minimum

infiltration rate, which is a stable and characteristic o f this soil type, the saturated hydrauhc conductivity (Figure 3.1, b). The bounding values of this set of curves in known as the infiltration envelope. This relationship is expressed as infiltration as a function of time in a number of well known equations such as those of Kostiakov (1932), Green and Ampt (1911) Horton ( 1940) and Phillips (1957); some empirical, and some mechanistic models of infiltration.

Figure 3.1

Standard model of infiltration applicable to temperate environments

a) The stages in the infiltration process for an idealised infiltration curve; pre-ponding infiltration until the ponding point is achieved resulting from an ever diminishing soil water storage capacity, the excess rainfall generating runoff, with the balance infiltrating at a declining rate minus surface water detention due to surface roughness, ‘non-pond’ infiltration. The ratio of runoff:infiltration increases

asymptotically until the infiltration rate equals the saturated hydraulic conductivity, a characteristic property of the soil. (Bowyer-Bower 1989)

b) An idealised infiltration envelope, characteristic for a given soil; irrespective of the intensity of the rainfall, a final, common steady state infiltration rate equivalent to the saturated hydraulic conductivity of the soil would eventually be reached, albeit following a curve the rate of decline of which is proportional to the rainfall intensity. (Smith 1972)

INFILTRATION CHARACTER IN RELATION TO THE ‘PONDING POINT

P r e - p o n d i n g i n f il t r a t i o n P o s t - p o n d i n g or rain p o n d i n f il tr a tio n R a in f a ll r a t e ( m m tir*^) P o n d i n g p o i n t I n f i l t r a t io n e x c e s s R u n o f f ) ' S a t u r a t e d c o n d u c t i v i t y (Ks a t m m h r ' h N o n - p o n d infiltrat io n T im e ( h r s )

THE INFILTRATION ENVELOPE

• P o n d i n g p o i n t s F L O O D E D IN F IL T R A T IO N P R E - P O N D I N G IN F IL T R A T IO N T H E IN FIL T R A T IO N E N V E L O P E P O N D IN G IN F IL T R A T IO N S A T U RA TE D C O N D U C T IV I T Y ( K s a t ) Ri"' N O N P O N D I N G INFILTRATION 10 20 30 40 50 60 70 80 90 100 T im e ( m i m ) --- ►

There is much debate about the conditions under which these equations are valid, but this will not be addressed here apart from examining the relevance to crusting soils. Horton (1940) was the first to recognise the importance o f surface features on infiltration and to model their infiltration. His infiltration equation often accurately describes the infiltration decay function commonly observed in the field with such soils in this empirical model, with the exponent alpha (which will be referred to in this study as ‘Horton’s alpha’, which is proportional to the rate of decay),

proving to be highly correlated in the present study on crusted soils in Baringo to other infiltration parameters. A number of influential models of infiltration into sealing soils are based on the Horton equation.

Green and Ampt (1911), on the other hand, produced an equation based on a mechanistic model

o f infiltration incorporating a number of simplifying assumptions, mainly that the soil in the wetted region has constant hydraulic properties and that the matric potential head at the wetting front is constant. Phillip, in turn, (1957) showed that the Green and Ampt equation is a special case of the general analytical solution of infiltration from a ponded surface into an infinitely deep homogeneous soil, even though it was formulated before the development o f the Richards equation. Many models o f infiltration into sealing soils are also based on the Green and Ampt equation.

(3.4.2)