Most of the land area selected for growing tea in the hilly states of North East India except the plains of Assam is located on hill slopes or on undulating topography, though occasionally there may be some areas having very steep slope. In other countries tea is grown on hill slopes as well. In such areas, all the soils are not equally deep and stable. Heavy rainfall at least during a part of the year, is a characteristic feature of most of the tea growing region. A combination of these factors is likely to lead to soil erosion, when the land is cleared for planting tea. In the slopping terrain where the soil is not sufficiently deep, heavy and continuous rain can cause land slide as is seen in the existing tea plantations of Darjeeling, India.
Thus, safe disposal of water from such areas is a perquisite for soil conservation in tea plantation. Flat and gently sloping land may also pose serious problems of the disposal of surplus water under the conditions of heavy rain. During the rainy months, water logging, high water table and occasional flooding in most of the tea areas of Assam pose some serious problems.
One of the most important factors responsible for soil erosion, land slide, poor drainage in different regions of North East India’s tea growing areas is the lack of catchment planning. In other words, planting of tea, construction of roads, and layout of drains in old tea plantations are not according to topography.
As topography dictates water movement on the soil surface and to a certain extent below the soil surface, conservation of soil and water, safe disposal of excess water, and good access can only be achieved correctly when the layout of a tea plantation is based on the topography. It is, therefore, emphasized that planting of tea in any new areas should be based on catchment planning for maximum sustained production which in turn is dependent on the best possible use of all assets.
A catchment is a self contained unit so far as water collection and disposal is concerned. It is bounded by ridges or high land and usually it contains a main outlet for out flow of water. The first requisite of catchment planning is to divide the proposed area into major natural catchments followed by further sub-division into minor natural catchments. Subsequently, these minor catchments are sub-divided by man-made constructions such as roads, paths and graded contour drains so that each unit becomes a catchment.
The catchment can be identified from the knowledge of the topography of the area. A complete level survey, designed to pick out all the topographical features is necessary for catchment planning. The survey and preparation of survey map must be done by some competent surveyor. It is suggested that the scale used should in no case be less than 25 cm to a kilometer. For flat land and for areas of less than 240 ha, the scale should be 50 cm to a kilometer. This is due to the reason that the larger the scale the clearer is the topography.
The topography of an area becomes obvious from the contour lines drawn on the map. A contour line is a line drawn on a map connecting all those points on the ground which are at the same distance above (or below) a specific datum, usually, mean sea level. These lines are numbered, and therefore, show the degree and extent of rise and fall of the land surveyed.