HeatDrought_I_Notes

HeatDrought_I_Notes - 1 Drought is very difficult to define...

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Drought is very difficult to define and it has many operational definitions, which we will get to later in this lecture. Simply put, drought is a deficiency in precipitation over an extended period of time. In other words, it occurs when a location is in an extended state of water deficit. Contrary to what most might think, drought is a natural and recurrent feature of climate. Although it is primarily controlled by precipitation, temperature is also an important variable when examining drought conditions. Anomalously high temperatures, for example, typically increase the water demand for plants and trees. Thus, higher than normal temperatures during dry conditions can amplify or accelerate the effects of drought for a given location. Conversely, lower than normal temperatures during dry conditions lower the water demand for plants. This can slow down the effects of drought for a given location and perhaps buy enough time for water to be put into the system before major impacts are observed. The above image shows a plot of land that is experiencing drought conditions. The lack of moisture in the ground has led to the development of a fractal-like cracking pattern that is indicative of extended periods of dryness in the area. 2
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As mentioned in the above slide, drought is difficult to define because the variables that control drought vary spatially from one location to another. For example, Baton Rouge, Louisiana averages a little over 60 inches of precipitation each year. In El Paso, an average of only 9-11 inches of precipitation is received in a year. Therefore, it is important that one considers values that are departures from normal when assessing drought conditions for a given location. There are other issues as well. For instance, some crops are capable of withstanding brief dry spells better that other crops. Hence, an extended period of water deficit at a given location may have more of an impact on one type of crop and may not impact other crops at all. 3
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The hydrological cycle, or water cycle, refers to the constant exchange of water with the Earth’s Hydrosphere (that part of the Earth that contains water). The cycle itself is driven by the solar energy balance. The cycle includes the movement of water - from the atmosphere to the surface (precipitation), from the surface to the atmosphere (evaporation and transpiration from plants), through the atmosphere (horizontal transport of water vapor) and through the surface (infiltration of water into the surface, percolation of water through the surface into groundwater and runoff of water through the surface). 4
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In order for us to fully understand what drought is, it is important that we have an understanding of the components that control it. The water balance equation is a useful tool for scientists as it provides a means by which one can measure and/or estimate the balance of water inputs to a system and water outputs from a system. A system could refer to a column of soil, a farmer’s crop field, or a natural grassland. By measuring the components
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This note was uploaded on 04/03/2010 for the course DSM 2000 taught by Professor Romolo during the Spring '08 term at LSU.

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HeatDrought_I_Notes - 1 Drought is very difficult to define...

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