OGE 121 : BACKGROUND TO PHYSICAL RESOURCES --- THE OPEN UNIVERSITY OF TANZANIA.

Q. with   specific  examples , explain   socio-economic   impacts  of   drought.

ANSWER

Drought is an extended period when a region receives a deficiency in its water supply, whether atmospheric, surface or ground water. A drought can last for months or years, or may be declared after as few as 15 days

Consequences

Periods of droughts can have significant environmental, agricultural, health, economic and social consequences. The effect varies according to vulnerability. For example, subsistence farmers are more likely to migrate during drought because they do not have alternative food sources. Areas with populations that depend on as a major food source are more vulnerable to famine.

Drought can also reduce water quality^, because lower water flows reduce dilution of pollutants and increase contamination of remaining water sources. Common consequences of drought include:

• Diminished crop growth or yield productions and carrying capacity for livestock
• Dust bowls, themselves a sign of erosion, which further erode the landscape
• Dust storms, when drought hits an area suffering from desertification and erosion
• Famine due to lack of water for irrigation
• Habitat damage, affecting both terrestrial and aquatic wildlife^.
• Hunger, drought provides too little water to support food crops.
• Malnutrition, dehydration and related diseases
• Mass migration, resulting in internal displacement and international refugees
• Reduced electricity production due to reduced water flow through hydroelectric dams
• Shortages of water for industrial user^.
• Snake migration, which results in snakebite
• Social unrest
• War over natural resources, including water and food
• Wildfires, such as Australian bushfires, are more common during times of drought and even death of people
• Exposure and oxidation of acid sulfate soils due to falling surface and groundwater levels

OGE 121 : BACKGROUND TO PHYSICAL RESSOURCES -- THE OPEN UNIVERSITY OF TANZANIA.

Q.  Describe  the   main  processes   involved  in  mechanical   weathering.

Mechanical Weathering

Let's say you're a big rock living in a national park. For the most part, life is good as you spend your days living outside in the natural environment. You get warmed by the sun on a daily basis and get cleaned by the gentle rains that occasionally fall on you.

However, over time, the constant exposure to forces, such as wind, water, cold and heat, take their toll on your rocky structure. These forces lead to mechanical weathering, which is defined as the physical breakdown of rock by environmental factors. In this lesson, we will take a look at how this process works and see examples of the different types of mechanical weathering.

The Process of Mechanical Weathering

Mechanical weathering is a physical process that is constantly happening in nature because nature is always active, even if this activity is happening slower than our senses can detect. For example, we feel when temperatures fluctuate from daytime to nighttime, but we can't see a rock expanding and contracting due to this temperature fluctuation. However, this unperceivable movement does occur and it weakens the rock, eventually causing it to fracture and break down. This is just one example of how mechanical weathering happens.

OGE 121 : BACKGROUND TO PHYSICAL RESOURCES--THE OPEN UNIVERSITY OF TANZANIA.

Q.  How   does  Mass  Wasting  differ   from  Soil  Erosion ?

Mass Wasting

If a rock slides off of a mountain and no one is around to hear it, does it make a sound? Well, I don't know about the noise this activity would create, but I do know that mountains erode and that rocks and debris can slide and fall down mountain slopes in massive amounts. In this lesson, you will learn about a process called mass wasting and the factors that cause this movement of material.

Mass wasting, which is sometimes called mass movement or slope movement, is defined as the large movement of rock, soil and debris downward due to the force of gravity. In other words, the earth's outer crust is being 'wasted' away on a 'massive' scale and falling to lower elevations.

Mass wasting is a type of erosion and it is capable of making big changes to the side of a mountain. These changes can happen suddenly, as in one minute the rock is there and the next it is gone, or it can happen more slowly over time. You might think of this process as a landslide, and this term is sometimes used interchangeably with mass wasting. However, the term landslide is a bit limiting and does not allow for a description of the many different triggers and types of erosion that can happen on this large of a scale.

Causes of Mass Wasting

Now, we mentioned that mass wasting is mainly due to gravity. So we see that mountains have an ongoing tug-of-war with gravity. Gravity is constantly trying to pull rock and debris down the slope of a mountain. At the same time, the resistive forces of the mountain, including the cohesive strength and internal friction between the materials, referred to as the mountain's shear strength, constantly pulls back against gravity.

The shear strength works to maintain the slope's stability and keep the materials in place. This is a lot like a mountain climber gripping onto the side of a mountain and resisting gravity. The climber uses his grip strength to resist gravity, like the mountain uses its shear strength.

With this understanding, we see that the causes of mass wasting occur when gravitational force overcomes the resistive forces of the mountain. And, since gravitational pull is always constant, then we see that mass wasting occurs when something changes the mountain's ability to resist gravity.

For instance, an increased slope steepness increases mass wasting simply because the gravitational force acting on a steep slope is greater than the force acting on a gentle slope. Increasing the steepness of a slope is one way man can increase mass wasting. For example, if a road crew cuts away a slope to make room for a new road but makes the angle of the slope too steep, the slope will be prone to mass wasting, and you will want to cross your fingers when you drive past this steep slope so no rocks or debris fall on your car!

Increased water is another factor that plays an important role in mass wasting. Water can wash away small particles that help keep the mountainside intact. This is similar to what happens when a wave comes ashore and washes away a sandcastle. The abundant water breaks apart the small sand particles and destroys the structural stability of the castle you spent the afternoon building.

If an area has decreased vegetation, it will be more prone to mass wasting. Vegetation stabilizes soil particles on the surface and anchors soil under the surface through its root system. This is much like comparing two sand dunes on a beach. If one sand dune has grasses growing on it, it will resist the erosion of water and wind better than a sand dune without vegetation.

Another factor that plays a role in mass wasting is earthquakes. The violent shaking that occurs in a region where an earthquake takes place has the ability to break off sections of mountains or hills, causing them to slide down the slope.

What Is Soil Erosion?

When things erode, they wear away due to some force acting on them. Just look at any coastline and you will notice how the constant pounding force from wind and waves causes erosion of the rocky structures, leaving behind all kinds of interesting cliffs, caves and structures. Soil is not immune to erosion, and like rocks along a coastline, soil can erode due to the effects of forces, such as water, wind and farming practices. In this lesson, we will learn about soil erosion and the factors that cause it.

Soil is naturally created when small pieces of weathered rocks and minerals mix with organic materials from decaying plants and animals. Soil creation is a slow process, taking many years. However, the soil that is created is constantly subjected to natural and manmade forces that disrupt it.

Soil erosion is defined as the wearing away of topsoil. Topsoil is the top layer of soil and is the most fertile because it contains the most organic, nutrient-rich materials. Therefore, this is the layer that farmers want to protect for growing their crops and ranchers want to protect for growing grasses for their cattle to graze on.

Water Erosion and Surface Water Runoff

One of the main causes of soil erosion is water erosion, which is the loss of topsoil due to water. Raindrops fall directly on topsoil. The impact of the raindrops loosens the material bonding it together, allowing small fragments to detach. If the rainfall continues, water gathers on the ground, causing water flow on the land surface, known as surface water runoff. This runoff carries the detached soil materials away and deposits them elsewhere.

There are some conditions that can accentuate surface water runoff and therefore soil erosion. For example, if the land is sloped, there is a greater potential for soil erosion due to the simple fact that gravity pulls the water and soil materials down the slope. Also, water will have an easier time running across the surface, carrying topsoil with it, if the ground is already saturated due to heavy rains or the soil lacks vegetation to keep the soil in place.