Module 4 Narration: Runoff and Recharge Timing Key Narrative Snapshots a) 00:44 How does groundwater recharge occur? How do streams form? These processes are closely linked. Lets first investigate how water soaks into the ground and later come back to the question of how this is related to stream flow. Video By the end of this module, you should be able to explain: Where groundwater comes from (or how recharge occurs), Why urban areas (with mostly impermeable soils) generate so much runoff, What factors related to precipitation and infiltration cause a flash flood, and Why gardeners prefer a slow, steady rain rather than a heavy downpour. b) 00:33 Our simplified water cycle shows that when it rains, water can evaporate from the surface, be taken up by plants, infiltrate into the ground or runoff from the surface. So infiltration moves water into the ground, where it moves downward and recharges or adds to groundwater and runoff happens as water moves downhill across the surface, where it contributes to stream flow. In this module, we will illustrate some common ways runoff, recharge and streamflow are related. c) 00:20 not used Imagine if we were able to open up a large fissure or trench in the ground so that we could look at a sub-surface cross-section of soil. Near the surface, you might see roots or animal burrows but as you move deeper all you see is a poorly sorted mixture of rocks and soil. Further down, you are likely to encounter groundwater. Repeat part of water cycle Pan across terrain and down x- section 1
d) 00:55 Groundwater is water that occurs in the saturated zone an area underground where all the spaces between the soil and rock are full of water. All groundwater initially starts as rainfall, snowfall or stream flow that infiltrates through the shallow soil. Water moves downward through the cracks in rocks and through tiny air-filled spaces between soil grains called pores. Eventually the water reaches an area below the surface where all the pores and cracks are filled with water. This is called the saturated zone. The top limit of the saturated zone is called a water table. The infiltrating water merges with the water table and begins to move with it at this point we consider this water to have become groundwater. e) 00:10 Just above the water table is a partially saturated zone created by water pulled up by capillary forces that act in the smallest cracks and spaces between soil grains. f) 00:36 Just as water can move through the pore space between the grains of rock in sediments, water can move through faults or fractures in rock. At a regional scale, water movement is controlled by underground faults in the rocks. Faulting often occurs in repeating and intersecting sets called joints. Faults and joints are widespread in the underlying basement rocks that makeup most mountain ranges. Zoom to capillary fringe g) 00:30 Lets begin our investigation by considering a regional view of a mountain range and adjoining basin or valley do you think groundwater recharge takes place uniformly everywhere in this picture? (pause) For groundwater recharge to occur, you need both the availability of water and the ability of the water to easily soak into the ground. Where is there the greatest source of water? (pause) Yes as we have seen, precipitation tends to be greater at higher elevations. h) 00:32 Now ask yourself where you think water might soak into the ground most easily? (pause) If you have ever been to the ocean, perhaps you noticed how quickly water soaks into beach sand. Valleys are another place where well-sorted sands and gravels make good recharge zones. In fact, there are four basic processes that dominate regional recharge. Each of these takes place in a different part of the watershed, from the highest elevations, down the mountain front and then along the valley floor. Overview of mountains and valley fade in precipitation color map Start with video of beach? Then slowly reveal x-section 2
i) 00:34 Recharge that occurs near the top of mountains is called mountain block recharge because of the way joints, or intersecting fractures in the rocks, divide mountains into large blocks. While recharge is slow, the ready availability of water from winter snowmelt and the large number of fractures allows water to slowly move down interconnected fracture systems to basin aquifers below. How slow is it? Mountain block recharge may take hundreds of years to move through faults, into a regional aquifer, that might eventually help recharge a valley stream. j) 00:38 Recharge that occurs closer to the base of the mountain is called mountain front recharge. The mountain front is often made of a series of parallel faults that separate areas of uplift and tilting from the nearby sediment-filled valleys. This is often the most prolific zone of recharge as mountain streams first encounter deep, coarse gravel sediments that readily transmit water down into deeper aquifers or water saturated sediments. Mountain front recharge may take decades to move through the groundwater system before it eventually is used or contributes to the base flow of a valley stream. k) 00:44 Recharge that occurs along the valley floor, primarily from widespread rainfall, is called valley floor or diffuse recharge. This form of basin recharge is extremely hard to estimate in semi-arid environments because the amount of water lost due to evaporation and transpiration is almost equal to the amount of precipitation that falls here. Still, because western basins represent such a large surface area, in wet years or during wet seasons, valley recharge can be significant. Because this water is near the surface and much closer to stream channels, it may only be in the groundwater system for a few years before contributing to the base flow of a valley stream. l) 00:43 We keep talking about how shallow aquifers contribute base flow to valley streams but how does this really happen? (pause) Streams and riparian zones in semi-arid areas are often in delicate balance with their surface and groundwater sources. When the system is in equilibrium with its surroundings, riparian streams flow due to a combination of inputs including upstream runoff and groundwater recharge. The groundwater component of stream flow is known as base flow. This cross-section represents a gaining stream, where a relatively high water table (arrow) allows groundwater to supplement stream flow. Zoom to Mountain block section more snow then allow this to melt as water soaks in Zoom to Mountain front section Zoom to Valley section Gaining stream 3
m) 00:26 During floods, water moves into adjacent banks (arrows) to become what we call bank storage. This shallow groundwater acts both to reduce the flood peak and maintain stream flow long after the flood has passed and the river has returned to its channel. Thus, bank storage acts like a sponge to soak up excess water and then slowly release it back into the stream. n) 00:21 During extended dry periods, most streams do not have enough upstream recharge to maintain this loss or recharge to the groundwater system and they can go dry. If the water table drops far enough, many trees within the riparian corridor, particularly Cottonwoods, will die, as their roots do not reach down to the water table. o) 00:24 As the channel refills with spring runoff, recharge occurs along streams and dry or ephemeral channels. Streambeds also tend to have many coarse sands and gravels that make it easy for water to move into the ground. Where the groundwater table lies much below the surface, there tends to be flow from the stream into the ground so this section of a river is called a losing stream. p) 00:36 Even under normal riparian conditions, if we pump enough water out of shallow riparian aquifers, streams can quickly become losing streams and the water table will drop below the point most riparian trees can survive. Western water law often characterizes water as either surface water or groundwater but it should be clear that both are intimately linked, particularly close to a stream as groundwater pumping lowers the water table, a stream can become losing and much of the pumped water is actually drawn from the stream. q) 00:25 There are many examples of this occurring throughout Arizona. One of the better photographic comparisons that we have is on the Santa Cruz river near Tucson. In the first photo, taken in 1942, the river supports a healthy riparian zone. In the second photo, taken in 1989, the trees are gone exposing a wide sandy channel. r) 00:28 Let s take a closer look at what determines how much rainfall becomes infiltration and how much becomes runoff. In general infiltration is controlled or limited by the permeability of the soil or surface cover. Permeability refers to the ability of a material to allow water to move through its pores. Flooding Bank storage Drought conditions Channel recharge Losing stream Normal, then Effect of pumping Santa Cruz Pics Just Passing through video? 4
y) 00:16 When rain falls at a rate faster than it can soak into the soil, the excess builds up and begins to move across the surface without sinking in. This water is called runoff. As permeability increases and more water infiltrates, runoff decreases. Animation of runoff in summer rain s) 00:32 Lets consider a coarse grained soil that contains gravel and a fine grained soil that contains silt or clay. How do you think soil grain size affects permeability? (pause) Which soil will produce the greatest runoff? (pause) The soil with the larger grain size tends to have greater permeability and thus produces less runoff. Similarly, the one with the smaller grain size tends to have less permeability and thus produces more runoff. t) 00:13 In contrast, solid rock, pavement and the rooftop of your home are said to be impermeable, meaning that they do not allow water to infiltrate, and all the rain that falls on these surfaces will become runoff. u) 00:37 What happens if you walk in the same place across a grass lawn or dirt lot? Besides killing the grass you are compacting the soil. Soils become less permeable as they not used are compacted, which means they will produce more runoff and allow less water to infiltrate. This is one reason you sometimes see machines punching holes into grass fields around playgrounds they are aerating the soil and reversing the effects of compaction. Many other natural and man-made processes can increase or decrease a soil s permeability such as plowing and animal burrowing. v) 00:25 Soils also become less permeable as they become saturated. This is a common occurrence in the southwest during the summer the first monsoon rains infiltrate into the dry soil and generate little runoff. As more rain falls, the soil pores fill with water and the rate of infiltration decreases. Since the water can no longer move downward, more of the rainfall becomes runoff. *Animation of rainfall on two flat soil blocks fine & coarse grained Animation of water flowing off a roof Animation of a tire track Animation of top-most pores filling up w) 00:17 not used In part, what happens to precipitation when it reaches the ground depends on how fast it falls. The water coming from a sudden thunderstorm will behave quite differently than water from a slow, gentle rain or water from snow slowly melting. Dry section (same as below) 5
x) 00:28 not used When water falls on soil, it soaks in and moves downward. As we have seen, the speed of this infiltration is affected by many things, including: the type of soil, the abundance and type of plants growing in the area, and the slope of the land. As long as the rate at which the rain falls is slower than the rate of infiltration, water falling on the surface will continue to infiltrate into the soil. Animation of infiltration in (slow) winter rain z) 00:26 Soils in semi-arid regions like the Verde River watershed tend to have very low infiltration rates, therefore runoff starts to happen fairly soon after a rain event begins. When a heavy, intense, localized rain event occurs, as often happens during the summer monsoon season, a lot of runoff can be generated very quickly! Sometimes this can lead to a flash flood. aa) 00:20 So now you should have a good idea of how recharge occurs and what conditions affect infiltration and runoff. All other things being equal, gentle, steady winter rains produce the most infiltration and the lowest runoff while heavy, intense summer storms produce the highest runoff. Video of a flash flood bb) 00:35 cc) 00:16 We have saved the easiest question for last. How do streams form is something that most of us have seen at least on a small scale. We probably were not thinking about what happens underground about permeability and saturation but it is clear that as water begins to pool on the surface during a heavy rain, it naturally fills depressions and follows the path of least resistance as gravity draws it to the lowest elevation. Sometimes this is called overland flow because the water is not flowing in a channel yet. As more and more water comes together, well-defined rivulets and channels begin to form. Finally, as thousands of micro channels feed larger channels in well-defined valleys, the flow amount increases until you would call it a creek, stream or river. Animation of water pooling and beginning to coalesce Animation of many rivulets forming a creek 6
dd) 01:00 Scientists who study water in rivers and where it has come from use chemical tracers to determine the age and likely origin of the water. It turns out that the water that you see in a stream arrives there via three different pathways and each one can be characterized by the speed at which it happens. Surface runoff is the fastest method because water quickly runs around anything in its path. Water that infiltrates into shallow soil can only move so fast through the small pore spaces between soil grains. Still, this pathway is faster than water that infiltrates into deeper soil and rocks, a process called recharge, which we described at the beginning of this module. Although infiltration is initially vertical, as this water encounters deeper saturated soil or impermeable rocks it will move sideways, or laterally, toward the lowest point. Animation of fast, medium and slow water pathways to a stream 7