15 May SHIFTING LANDUSE AND STARVING AQUIFERS – A BANE OF WAYANAD
SHIFTING LANDUSE AND STARVING AQUIFERS – A BANE OF WAYANAD
Land use shifts, for e.g., from agricultural to horticulturalurban, generally driven by developmental activities like urbanization, new infrastructure like bus terminals, schools and colleges, apartment complexes and garden houses, industrial parks, power switching yards, water treatment and sewage pumping stations etc., are common place all over the world. Generally speaking, farmland along the perimeter of urban centers is the first causality, whenever any expansion of the city is conceived and implemented. Yet another landuse shift is with chiefly regard to change in the type of crop/s cultivated. In the long run, while urbanization reduces the cropped area, change in crops and hence treatment of the soil, affects, the groundwater regimen. Further, there had been great shifts in the type of crops planted due to the classical supply-and-demand forces. Switch from rice paddy farming to horticulture like plantain, ginger root, arecanut, vegetables and even flowering plants is a case in point. Demographics and economics are forcing such changes. In what follows, I shall argue that the water scarcity in Wayand is a direct outcome from the changed agricultural practices and crops cultivated in this region. These causative factors can be extended to tracts out side Wayanad also.
Interestingly, these days though average Keralite does generally know the basic reasons for water scarcity and water shortage in the state, which otherwise is a tract blessed with 41 west flowing and 2 east flowing rivers and two monsoons, viz., SW and NE, annually dumping nearly 3000 mm of annual rainfall. The state of Kerala being stuck on the western slopes of the Western ghats is physiographically very unique with a land area that is relatively steep from its eastern edge on the ghats to the Laccadive sea shoreline in the west. The surface slope of say 3000 in 50,000 enables a quick rush of surface flow or run off to reach the ocean in a day or two of its incidence. Residence time of surface water is very critical in so far as the recharge of the various hard rock, gruss and Tertiary aquifers are concerned. In fact, water needs in agricultural, domestic and small scale industrial sector are largely supported by groundwater or the rain water that is stored in rock layers in the upper parts of the earths crust.
Generally, in Kerala, surface cover in coastal land (el.=<7.5 m) and midland (el.=7.5-75.0 m) are modern beach alluviam, sedimentary rocks of Tertiary age and the gruss or weathered upper part of the crystalline metamorphic basement (like gneisses and charnockites). Highland (el.= >75.0 m) encompassing the Western ghats are composed of a variety of crystalline metamorphic rocks (gneisses, charnockites etc.), intrusive granite bodies and basic intrusives equivalent of Deccan trap activity.
A large number lineaments classified as major, intermediate and minor) are well documented through the use of aerial photos and satellite imageries. And many geoscientists are of the view that major lineaments indeed are deep-seated fractures and infrequently are reactivated causing earth tremors or milder quakes. Based on their orientation, lineaments are grouped under three sets (viz., NNW-SSE, NW-SE and N-S) roughly coinciding with the regional structure reported earlier.
Drainage net of major rivers reflects the influence of major and sometimes intermediate lineaments, in that the master stream and tributary channels succinctly reflect the trend of controlling lineaments. More over some lineaments are marked by or associated with shear zones which are excellent conduits of recharge of water to aquifers.
Structure of recharge of aquifer: A primer
A key to the recharge of aquifer/s is availability of sufficient rain, presence of a canopy of vegetation, under brush, litter of decaying vegetative matter, soil and the numerous weak or porous zones in the basement rock below soil. These factors work in a manner that is nearly imperfectly known to the lay citizen. In order to understand recharge process of an aquifer, one must follow the path of a rain drop. As drop size varies considerably, the force of impact too. Larger the drop size, higher the degree of impact. Drops hit vegetative cover, soil, rock or others at the terminal velocity.
As far as rock surfaces are concerned, damage to it is infinitesimally low, unless the constituent mineral particles are in a state of disaggregation due to weathering. In the latter case, force of falling rain drop will be good enough to dislodge the minerals of the rock. But, at the same time, impact may be sufficient enough to displace soil particles and alter the soil fabric or soil structure adversely. Water drops hitting the soil will also fill pore spaces to saturation. However, once the pores are saturated, water will start to flow down slope initially as sheet flow or as sheet of water and then through rills and into first order streams of the river net. Flowing water will also carry soil particles either as suspended load or as bed-load along with a very low content of dissolved load of chemical ions.
Role of vegetation
Contrary to this, a litter of decaying vegetation on the soil, or grass, or a canopy of vegetation, soil on the ground is not directly exposed to rain drop impact as the drops of water are intercepted by the vegetative cover and slowly cascades down to the soil layer, taking a longer period of time to reach the soil. Remember the folklore – rain rains for a day, but canopy rains for four days, which aptly states the role of tree canopy in decelerating the fall speed of rain drops.
Thus water drops finally hitting the soil cover would have lost all momentum and erosive impact power would trickle down through the layer of litter. Larger particles of water would gather together and flow downward through the leaf tips and cascades downward invariably without inflicting any damage to soil cover especially its structure. In fact such a (cascading) trajectory of water drops virtually lengthens several times the duration or time taken by water drop to reach the ground. On the ground due to a cover of litter (a carpet of dead leaves and twigs) water drops do not directly come in contact with soil surface. The cascading flow and litter (acting like a sponge) tremendously increases the residence time of water on the ground enabling excellent opportunities for recharging the soil and thus the aquifers.
Water drops hitting steep tree branches that join the trunk, propel themselves down the trunk toward bottom and flows radially outward eroding the soil cover and exposing the large roots.
Foliage cover, very much like a sheet of cotton cloth allowing raindrops to slowly descent down and disappear in litter covered ground and soil below it, is much like a moist cloth. But a surface wpith out vegetative cover is very much like a stretched sheet of plastic propelling water outward and away.
Role of humans
In fact, most of the time role of humans in triggering, modifying or accelerating earth’s suface processes is not fully appreciated either by human’s themselves or by the societal leaders. Only group that understood the connection is the scientific community. Land use and land-cover changes undoubtedly are areas subjected to a great deal of influence by human actions or anthropogenic interventions. In what follows, I shall demonstrate the consequences of land use changes on aquifers by taking an example of Wayand dist, Kerala.
Wayanad (refined from wayal nadu or rice paddy region), siting at the SW edge of Mysore plateau of Karnataka, is deeply dissected, dominantly by Kabini R. (a tributary of Cauvery R.) and its tributaries. Geologically, the rock formations mapped in this tract are crystalline metamorphic rocks of Pre-Cambrian age and the river alluviam filling the valleys. Though sedimentary rocks of Paleozoic and late Mesozoic are absent, but late cretaceous to Tertiary or even younger Laterite is noticed as low gently rolling, hills supporting excellent coffee and tea plantations. While valley fill of Quaternary alluviam is ubiquitous as relatively even tracts extending from either slopes to toe of hills, its distribution is asymmetric with respect to the channel.
Until very recently rice paddy was the chief crop raised twice annually, in the alluvial tract, fashioned into terraces of large and small extent. However, many factors like scarcity and rising wage of farm labour, rising cost of inputs like seed, fertilizer, pesticide and unattractive and unstable and relatively low price of paddy have weaned away an average Wayanad farmer to other more lucrative areas like, plantain, ginger, arecanut and vegetables d forced many farmers to quit paddy farming and go into other crops like ginger, plantain, arecanut, vegetables etc. This transition into a new set of crops in majority of paddy farms, adversely affected the geo-hydrologic make up of Wayanad district, resulting in near drought conditions and warranted ferrying of water to large segment of rural and tribal population.