The word arid means dry. The dry land is called arid land. This problem is called aridity. Arid climate is defined as the climate in which precipitation is less than the evapotranspiration in most part of the year. Deficiency of rainfall is a basic characteristic of the arid climate. Pakistan is an arid and semi-arid country. 68 million hectares of Pakistan faces aridity. The Potohar and Baluchistan plateau have arid lands. Many factors play role in aridity. These are:
- Seasonal distribution of rain fall: The average rain fall is less than 250 mm in Pakistan. Similarly, this rain is not distributed evenly through out the years. Raining takes place mostly during July to August. The land remains dry most of the part of the year. It causes aridity.
- Nature of soil type: Nature of soil also affects the aridity. Sandy soil has low water holding capacity. So most of the water runs away. It causes aridity. Organic matter and humus increase the water holding capacity of soil. But generally land of Pakistan has low organic contents.
- Evaporation rate: Evaporation rate also affect the aridity. Generally, high evaporation rate causes dryness of soil. It promotes aridity.
- Vegetation cover: The vegetation cover reduces the evaporation from the soil. It also increases the water holding capacity of the soil. Thus vegetation cover reduces aridity.
- Water holding capacity of soil: Aridity is inversely proportional to the water holding capacity of the soil. Generally, soil with clay or silt has more water holding capacity. Organic matter increases the water holding capacity of soil.
- Percolation and seepage rate: It has great effect in aridity. High percolation and seepage rate promote aridity. Soil becomes dry. Sandy soil has high percolation and seepage rate.
Underground water (water table): Low level of water table causes aridity. In this case, water does not come to the surface by capillary action. Therefore, plant cannot get water.
Strong and dry wind: Strong and dry wind increase evaporation rate. So moisture of the land is reduced. Therefore, land becomes arid.
Effects of aridity
Aridity has adverse affect on crop production. The productivity of arid land is far less. Most of the arid lands depend on rain for water. There is unequal distribution of rain throughout the year. So very few crops can be grown in these regions.
Control of aridity
Aridity is a natural phenomenon–. It is caused due to low and uneven rain. But there are certain methods which can reduce the affects of aridity. These are:
- Mulching: Soil must be covered with shallow cultivation. The stumps of the plants should not be removed after harvesting. They act as natural mulch and increase the water holding capacity of soil. Straw mulch is also affective in reducing evaporation of water.
- Ploughing and harrowing: Ploughing and harrowing break the capillary link of the lower soil with the surface soil. So it reduces seepage of water. It also increases the water holding capacity of soil.
- Eradication of weeds: Weeds cause !way.; transpiration. So they reduce the moisture contents of the soil. Weeds should be removed to control aridity.
- Crop rotation: Crops should be rotated continuously. The variation in requirement of water by different crops can keep balance in the soil.
- Addition of organic matter: Natural manure and litter can increase organic contents of the soil. Organic contents have great water holding capacity. So they reduce the aridity.