Unit 8 Lab. Part 1: GROUNDWATER, WATER TABLE, AND FLOW

Transcription

1 Unit 8 Lab Part 1: GROUNDWATER, WATER TABLE, AND FLOW Educational Outcomes: Groundwater is the subsurface water that completely fills the openings in Earth materials and is available to supply wells, springs, and streams. The water table is the upper surface of the groundwater in unconfined soil, sediment, or rock. If Earth materials are permeable (allow water to readily pass through), the groundwater may be tapped in usable quantities by wells. Most rural residences and many agricultural operations depend on such water supplies. Because groundwater is such an important resource, some wells are maintained not to pump water but to monitor the level of the water underground. Such monitoring wells are important in determining the availability of water supplies and their quality over time. Investigation: Nebraska is one of the states that contains the Ogallala or High Plains aquifer, an extremely important groundwater source for much of the middle US stretching from Nebraska to Texas. Monitoring wells have been used in various locations at different times throughout the several-state area in which the aquifer is located to determine the levels of the water table and changes in the aquifer.

2 1. Figure I is a map of ground surface elevations at some of these well sites in a small area of central Nebraska. Numbers next to dots on the map represent the elevations of the land surface above a horizontal surface 2000 feet above sea level as determined at well sites. That is, the two-digit number indicates the elevation in feet with 2000 subtracted for convenience. For example, "28" corresponds to an actual elevation of2028 feet above sea level. Locations of wells are given in terms of longitude and latitude as is common practice.

3 Examine the values plotted on the graph. The highest elevation plotted is This represents feet above sea level. This occurs at approximately ' 10" West longitude and 0 ' 20" North latitude. The lowest elevation plotted is ' This represents feet above sea level. This occurs at approximately 0 - ' 40" West longitude and ' 20" North latitude. 2. Draw contour lines (connecting points of equal elevation) in intervals of 5 feet that cover the range of elevations plotted on the figure. Thus, contour line values of, ' 2040, ' 2050, feet should appear on the map. Following the example given with the 2040-foot contour, draw a specific contour line through locations with the line's value while keeping places with lower elevations consistently on one side and places with higher values on the other side of the line. Based on the contour pattern revealed by your analysis, the land surface generally slopes downward from [(west to east) (east to west)]. Furthermore, based on the curvature of the contour lines, there is a west/east oriented [(ridge) (valley)] running across the middle of the field of values.

4 3. Figure 2 is a map of the same area of Nebraska with elevations of the water table in 1948 as measured in wells located at the dots shown. The elevation plotted was determined by subtracting the depth to the water in each well from the surface elevation. Again, these values are in feet above 2000, as was done in Figure I. The highest water table value in 1948 was at feet above sea level. This occurred at a well site in the [(northwest) (southwest) (northeast) (southeastè) portion of the area. The lowest value was at feet above sea level. This occurred at a well site in the [(northwest) (southwest) (northeast) (southeastè) portion of the area.

5 4. Draw contours lines of water table elevation in intervals of 5 feet that cover the elevations plotted on Figure 2. Thus, contour line values of 2010,,, 2030, feet would be used. The 2030-foot contour line has been drawn as an example. (Ignore the small arrow appearing on the contour line for now.) Based on the pattern shown by the contour lines, in general, the water table slopes downward from ((southwest to northeast) (southeast to northwest)]. Furthermore, based on the curvature of the contour lines, the water table exhibits a general southwest/northeast oriented ((ridge) (valleyè) across the field of values. 5. Comparing Figures I and 2, ground elevations along the western edge of the data area appear to reach as high as about ((2055) (2045) (2035È) feet above sea level. Water table elevation levels along this same edge reach as high as about [(2055) (2045) (2035)] feet above sea level. The depth from ground level to the water table is therefore approximately [0 (l0) (20)] feet along the western edge. Ground elevation along the eastern edge of the data field averages about [(2050) (2040) (2060)] feet above sea level. Water table elevation along this same data edge averaged, about [(2035) (2025) (2015)] feet above sea level. The depth from ground level to the water table is therefore approximately [ (0) (10) (20)] feet along the eastern edge. 6. The relatively uniform depth to the water table from the land surface throughout the map area indicates that in general the water table [(does) (does not)] parallel the ground surface above. Where the land surface is high, the elevation of the water table is [(high) (low)], and where the land surface is low, the elevation of the water table is [(high) (low)]. 7. Groundwater tends to flow under the influence of gravity, that is, "downhill". It will flow in the direction perpendicular to water-table contour lines and towards lower elevation. Water movement patterns relative to a contoured water table can be shown by drawing short arrows similar to the one already appearing on the 2030-foot contour line in Figure 2. The arrow shafts must always be drawn perpendicular to the contour line and the arrowhead should always point downhill to show the flow direction. Analyze the groundwater flow pattern on the map by drawing an arrow about every 1.5 cm along each contour line. Examine the arrows along the segment of the 2030-foot contour line between the 40 44' 30" and 40 46' 00" latitude lines. The groundwater flow direction in this region, as shown by your arrows, was generally toward the [(northwest) (southwest) (northeast) (southeast)]. 8. One use of well data is to determine whether or not the height of the water table is changing over time. It is of great importance to learn if more water is being withdrawn from an aquifer than is being replenished by natural recharge. Figure 3 provides information on the elevation of the water table in the same Nebraska

6 region in 1990 as determined from wells located at the dots shown (42 years after the Figure 2 data). Elevations are plotted above 2000 feet as was done in Figures 1 and 2. The highest plotted water table elevation in 1990 was - '- feet above sea level. This occurred at well sites in the [(northern) (southern) (eastern) (western)] portion of the study area. The lowest plotted value was feet above sea level, at well sites in the [(northern) (southern) (eastern) (western)] portion of the area. 9. Contour lines of water table elevation for 1990 have been drawn at intervals of 5 feet that cover the elevations plotted on the figure. Contour values of,,, feet appear on the map. Based on the contour pattern, in general, the water table slopes downward from [(east to west) (west to east)]. ~

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8 In 1990 there were contour lines defining the water table pattern. In 1948 there were contour lines defining the water table pattern. Therefore, in [(1948) (1990)] the range in elevation of the water table was less as shown by the fewer contour lines. In other words, in [(1948) (1990)] the surface of the water table was "flatter" although still following the same general pattern of the overlying land surface. 10. From 1948 to 1990, the water table contour lines, as exemplified by the and the middle portion of the 2020-foot lines, generally shifted [(eastward) (westward)). This indicates that at individual well sites the height of the water table generally dropped over the 42-year period. Therefore, a(n) [(addition) (reduction)] in the amount of water stored in the aquifer must have occurred during that period. Part 2: WATER TABLE UPS AND DOWNS Educational Outcomes: Groundwater is a most valuable natural resource. Its availability on a sustainable basis is critical for human habitation, agriculture, and industry in many places worldwide, especially in rural areas and arid and semi-arid locations with limited surface fresh water sources. Wise stewardship of the groundwater resource requires first determining its local availability and variability over time, some of which is due to general climatic factors such as precipitation. Investigation: Figure I is the record of the depth of the water table below ground level in a monitoring well near Trenton Dam in central Nebraska over the period from 1991 to The well is situated in a generally flat agricultural area where farmers and ranchers depend heavily on irrigation to supplement relatively light precipitation. (The area is located within the rain shadow of the Rocky Mountains.) The Ogallala (or High Plains) aquifer is the principal source of groundwater for irrigation. Changes in water level are shown in inches below a subsurface datum of 189 feet. Thus, a well water depth of feet is shown as -6 inches on the vertical scale. The horizontal scale is marked every month with labels every four months.

9 1. Examine the values plotted on the graph. The deepest water table level plotted is about 189 feet and [(5) (15)(25)] inches below the surface. This occurred in [(February)(June)(September)] of The shallowest water table level appears to be about 189 feet and [(1)(3)(5)]) inches below the surface. This occurred in approximately May of [(1991) (1994) (1998È). 2. The overall character of the water table level curve over the nearly eight-year period reported is: a) (basically a straight line (linear trend) that is increasing over the time period) b) (a curve that fluctuates UP possible relationship to time of year),c).(a curve that fluctuates UP and down (cyclic) each year). 3. The highest water table levels (least negative depth values) tend to occur during the period [(May) (July) (September to October)] each year while the lowest levels (greatest negative depths) tend to occur during the months of [(September to October)] each year. Each year's highest level is [(the same) (varies)] from year to year. Each year's lowest level is [(the same) (varies)] from year to year.. 4. During each year's water table fluctuations, the rising portion of the curve changes [(more) (less)] rapidly than the falling portion of the curve. This implies that the amount of the groundwater at this site generally decreases during the withdrawal period at a [( greater) (lesser)] rate than the rate at which it increases during the recovery phase.

10 Figure 2 is a graph with water table levels in the bottom portion as shown in Figure 1. A precipitation curve is drawn in the upper portion of Figure 2 by first by plotting monthly totals of precipitation at mid-month as measured at Trenton Dam and connecting adjacent points with lines. A potential evaporation curve is plotted in similar fashion. Potential evaporation amounts are only measured in the "warm" season (April though October) from changes in water level in a large pan exposed to the air. Precipitation (continuous solid line) represents an input of water to the land surface whereas potential evaporation (segmented dashed line) represents the maximum potential loss of water from the land surface. 6. At Trenton, Nebraska, a mid-latitude, mid-continental location, precipitation [ (is relatively uniform each month) (occurs mainly in spring and early summerè). Potential evaporation generally peaks during [(June- July) (August/September)].

11 Precipitation and potential evaporation [(do) (do not)) peak on average during the same several-month period of year. 7. Comparison of the precipitation and potential evaporation curves indicates that during the spring and summer growing season, evaporation is [(greater) (less)] than precipitation. This explains the need for irrigation to sustain the region's agriculture. 8. Evidence of irrigation during the spring and summer growing season is seen in the water level curve. During the withdrawal phase the water level drops dramatically, implying a(n) [(steady) (irregular)) rate of withdrawal-likely resulting from continuous pumping of large quantities of groundwater.

12 9. The data show that groundwater recovery typically occurs from [Ç(fall to spring) (spring to fall)]. The rise in the water table cannot be explained by the minimal precipitation that takes place during that time period. It is likely the recovery is due primarily to groundwater flow in the Ogallala aquifer from higher elevations to the west. 10. Over the nearly eight-year period of data shown in the graph, the overall trend of the water table level curve shows a [(steady rise) (steady fall) (neither steady rise nor fall)]. This indicates that, at least in this area of Nebraska, withdrawal of groundwater for irrigating crops at the same levels appears [(to be) (not to be)] sustainable for the near future without depleting the groundwater supply. Part 3: GROUNDWATER, WATER TABLE, AND FLOW In the first part of this investigation we saw how the water table tends to parallel the overlying topography and how long-term (multi-decadal) changes can take place in the depth to the water table most likely in response to groundwater withdrawal. The purpose of this portion of the investigation is to examine shorterterm variations in the water supply in the top 2 m (6.5 ft) of the soil-the water resource that is accessible by plant roots and is particularly important for agriculture. Soil porosity, the measure of the soil volume not occupied by solid particles, varies considerably. Compact soils may be 25% or so void space while wellgranulated soils may be more than 65% open space. Consequently, the potential capacities of soils to hold moisture are significant and can range widely.

13 Image 1 and Image 1 includes two maps prepared by the NOAA National Centers for Environmental Prediction (NCEP) showing estimated initial (current) soil moisture levels (in cm) and the forecasted change in soil moisture levels (in cm) over the subsequent 7 days. Soil moisture is defined as equivalent to the depth of liquid water contained in the top 2 m of the soil. Print a copy of Image 1. If you do not have a color printer, label locations with lowest and highest values by referring to the color coding seen on-screen. On the upper map, the highest U.S. values indicated are cm and the lowest U.S. values are cm. The lower map shows predicted changes generally ranging from between Ð4 and Ð2 cm up to +6 to +8 cm over the following 7 days. (To obtain the most recent of these maps, go to the Links Page, scroll down to "Terrestrial Information" and click on "Soil Moisture.") In Image 1, the upper map is a plot of the amount of liquid water in centimeters in the top 2 meters of soil valid for (date). The lower map is the forecasted change in soil moisture in centimeters over the subsequent 7 days ending on (date). 12. Several environmental factors govern the amount of soil moisture including precipitation, direct evaporation of water from the soil surface, the portion of surface water that infiltrates the ground versus the portion that runs off, and the uptake and transpiration of moisture by plants. We would generally expect more soil moisture where the weather is relatively [(wet) (dry)]. Questions 13 through 15 refer to the "Initial Soil Moisture" map, the upper map in Image According to the map, highest soil moisture values of 65 cm or greater are reported mostly in [(the Southwest) (the Ohio Valley and mid-atlantic states) (Nebraska and Kansas)] and lowest moisture values

14 of 35 cm or less are reported in states including [(Iowa and Missouri) (California, Nevada, and Arizona)]. The nation's wettest soils can be at least partially attributed to [(recent high rates of evaporation) (heavy rainfall from recent storms)]. 14. In a broad band from Texas to the Dakotas, soil moisture values generally [(increase) (decrease) (do not change)] from west to east. Knowing that the mountains of the West reduce the flow of atmospheric moisture in the prevailing winds from west to east, it appears that the increase in soil moisture towards the east must be due largely to atmospheric moisture originating in the [(northern North Pacific) (Gulf of Mexico)]. 15. Soil moisture generally [(increases) (decreases)] from west to east across Washington State. This geographical trend in soil moisture suggests that precipitation is greater to the [(west) (east)] on the [(windward) (leeward)] side of the mountains located in the western portion Washington. Questions 16 and 17 refer to the "Forecast Soil Moisture Change" map, the lower map in Image 1. This gives the predicted change in soil moisture levels in cm. 16. During the next 7 days, soil moisture is expected to generally [(increase) (decrease)] through much of the Central U.S. including most of Kansas, Nebraska, and Missouri. During the same period, soil moisture is expected to [(increase) (decrease)] over most of California and along the East Coast. From this forecast, we can infer that the principal storm track and accompanying precipitation probably [(is) (is not)] expected to be along the East Coast over the next 7 days. 17. Across most of peninsular Florida, the soil moisture content is forecast to [(increase by as much as 2 cm) (increase by more than 8 cm) (decrease by 2 to 4 cm)] over the next 7 days. This expected change in soil moisture is probably due to relatively [(wet)(dry)] weather conditions forecast for the next week. Higher temperatures during the same period would [(increase) (decrease)] the evaporation and transpiration rates. Therefore, higher temperatures would also contribute to the expected [(increase) (decrease)] in soil moisture. Because soil moisture is so closely associated with temperature and precipitation, you can compare the Forecast Soil Moisture Change map with temperature and precipitation forecasts for the same seven-day period via the "Soil Moisture" link under "Terrestrial Information" on the Links Page. When you access the "Soil Moisture" maps, links are provided at the top of the screen to the temperature and precipitation forecast maps. Call up the latest maps and attempt to determine the probable impacts of forecasted temperatures and precipitation on predicted soil moisture changes.