Geography midterm

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lecture9-17.docx.pdf

LOCAL WATER BUDGETS Following on the heels of lecture 8, the hydrologic cycle, we address the application of this knowledge to the local scale. Local water budgets examine the amount of water available for use in a specific area. As you will see discussed in the text (p. 228-235, 9th ed or p.222-230, 10th ed), there are many equations that we can use to estimate the amount of water that is available for a given region.

I want you to look at this task in three ways;

1. A water budget is the product of an area's ​rate of evapotranspiration ​and annual precipitation.

2. Water budgets examine ​annual relationships ​between evapotranspiration

and precipitation.

3. Natural vegetation adapts to these environmental constraints. EVAPOTRANSPIRATION ​Go to map p. 230, 9th ed or p. 223, 10th ed. Evapotranspiration ​is simply defined as the amount of water which is evaporated from the surface plus the amount of water which is transpired by plants. We can measure this amount of water lost by a given location. What is most important though, is that you recognize that both evaporation and transpiration take place in response to heat. In other words, ​Potential evapotranspiration ​simply reflects the amount of solar radiation a region receives. It tells us how much water is demanded by vegetation in these areas.

For clarity, please examine the potential evapotranspiration map. You will see in the highest areas, water transpired or evaporated in these regions (if an endless amount were available) are over 60 inches or more in the Mohave Desert and 54 -60 inches in the Southern tip of Texas. Lowest regions of Potential Evapotranspiration are found in Northern Canada (less than 18 inches) and Mountain regions, such as the Rockies or the Sierras.

Potential Transpiration illustrates the water that will be demanded by vegetation in these regions. For example, the POTET of Stanislaus County is shown as 30-36 inches. This is the amount of water demanded by vegetation in our region. The telling part of this discussion is to address the amount of water we actually receive in this region. On p. 229 (9th ed) or 223 (10th ed), the precipitation map shows us that Stanislaus County receives between 10-19 inches of rain per year. It becomes obvious that plants will have to adapt to our limited precipitation. They can do this by reducing their leaf size, or increasing their root system (tap roots) to increase underground moisture. The leaf size of plants determine the amount of water that is released through transpiration. Plants with large leaves (in the rainforest) transpire a large amount of water. Plants with small leaves (such as cactus-in the desert) transpire a small amount of water.

Examining runoff in greater detail allows us to see how the earth may store moisture and release such surplus for plants in times of greater evapotranspiration (in our area-summer and fall). Please look on the illustration to understand the following:

Precipitation​- shown as a blue line Soil moisture recharge​- as water is absorbed into the groundwater Surplus​- excess moisture stored in underground aquifers Soil Moisture Utilization​-water being extracted from underground aquifers Deficit​-all surplus moisture has been extracted

WATER BUDGETS ​(see page 233, 9th ed or page 226, 10th ed)

For each of the local water budgets illustrated on these two pages try to examine ​when​ the following occurs:​1. precip 2. temperature increases (this will show water demand/POTET)

Comparing these two elements will show you the ​seasonality ​of each regions needs.

● When do they receive precipitation? ● Does this precip become stored as soil moisture (recharge and surplus)? ● When does the demand for water increase (potet)? ● Is there enough moisture stored to supply this demand ?

Simply, to compare POTET and PRECIP, this will show us if our region has a surplus or deficit water budget. And when that surplus or deficit may occur.

LOCAL WATER BUDGETS ​- click here to connect to ​Stanislaus County water budget info.

USGS​ ​– website on water in the United States, the science behind water budget analysis (site and infolinks)

HUMAN MODIFICATION

Now, the question becomes is human diffusion of plants appropriate? Can we in good conscience, relocate large leaf plants and grasses, such as Kentucky blue grass (which requires between 30-42 inches a year) to Stanislaus County (which receives between 10-19 inches of precipitation per year)? In order to keep these plants alive, humans will have to add the minimum of 11-20 inches of water per year. Is this appropriate? We can modify the local water budget by relocating water from the Sierras to the Central Valley, but is this a good use of our water resources?