H&WRE_Unit 4 PPT.pptx

Hydrology and Water Resources Engineering Unit 4 PPT

1. UNIT 4 – Ground Water Hydrology Groundwater Hydrology Occurrence, movement and distribution of groundwater, aquifers – types, Specific Yield, Permeability, Storage coefficient, Transmissibility, Darcy’s Law. Well Hydraulics - Steady radial flow into well for confined and unconfined aquifers, Recuperation tests. Well constants.

2. What is Ground Water Hydrology? Groundwater Hydrology is the science of occurrence, distribution and movement of water below the surface of the earth. Total ground water potential is to be one third the capacity of oceans. The main source of ground water is precipitation. Other sources are water trapped in sedimentary rocks. Not suited for use

3. What is Ground Water Hydrology? Discharge from ground water occurs in two ways Natural way : occurs as flow in lakes, rivers, oceans and springs. Artificial way : pumpage from wells is the major artificial discharge.

4. What is Aquifer? Aquifers are permeable formations having structures which permit appreciable quantity of water to move through them under ordinary field conditions. Unconfined aquifer : water table serves as the surface of the zone of saturation. Confined aquifer : ground water is confined under pressure.

5. Aquifers

6. What is Aquifer? Flowing well: when a well penetrates a confined aquifer, water rises in the well to the level of local static pressure. Artesian well: the water level in a well is below the ground level, but is above the local water table.

7. What is Specific Yield? the specific field of an aquifer is defined as the ratio expressed as a percentage, of the volume of water which after being saturated can be expressed by gravity to its volume. specific yield = (Volume of water drained by gravity) / Total volume  

8. What coefficient of Transmissibility? The rate of flow of water through vertical strip of aquifer of unit width and extending the full saturation height under unit hydraulic gradient, at a temperature of 60 degrees F. coefficient of Transmissibility (T)=Bk

9. What Storage coefficient? The volume of water that an aquifer releases from or takes into storage per unit surface area of aquifer per unit change in the component of head normal to that surface

10. What Storage coefficient? The water yielding capacity can be expressed in terms of storage coefficient(S). Storage coefficient ranges between 0.00005 to 0.005. S can determined from pumping out tests.

11. Well Hydraulics Darcy’s law: for a laminar flow conditions in a saturated soil, the rate of flow, or the discharge per unit time is proportional to the hydraulic gradient. Q = kiA V= =ki  

12. Dupuit’s Theory Steady Radial flow into a well: When the well is pumped, water is removed from the aquifer and the water table is lowered resulting in a circular depression in the water table. This is called as cone of depression or drawdown curve

13. Dupuit’s Theory Unconfined Aquifer: Let r = radius of the well H = thickness of aquifer S = drawdown at the well h= depth of water in the well

14. Dupuit’s Theory Consider the origin of the coordinates at point O at the center of the well at its bottom. Let coordinates of the point P on drawdown curve be ( x,y ), from Darcy’s law Q=  

15. Dupuit’s Theory Q= =(2 Π x)x((y)=2 Π xy =hydraulic gradient at P =  

16. Dupuit’s Theory If the drawdown measured at the well is s=H-h and H = s+h . Then

17. Dupuit’s Theory Confined Aquifer: Consider the origin of the coordinates at point O at the center of the well at its bottom. Let coordinates of the point P on drawdown curve be ( x,y ), from Darcy’s law Q=  

18. Dupuit’s Theory Q= =(2 Π x)x((b)=2 Π xb =hydraulic gradient at P =  

19. Recuperation Test – yield of an open well In this test water level is depressed to any level below the normal and the pumping is stopped. The time takes for the water to recuperate to the normal level is noted. From the data, discharge from the well can be calculated as under.

20. Recuperation Test – yield of an open well Let aa = static water level in the well, before the pumping started. bb=water level in the well when the pumping is stopped. h1=depression head in the well when the pumping stopped. cc=water level in the well at a time T after the pumping stopped.

21. Recuperation Test – yield of an open well Let h2 = depression head in the well at the time T after the pumping stopped. h=depression head in the well at a time t after the pumping stopped. dh=decrease in depression head in a time dt. t,T =time in hours.

22. Recuperation Test – yield of an open well In a time t, reckoned from the instant of stopping the pump, the water level recuperates by (h1-h) meters. In a time dt after this, the head recuperates by value dh meters. Volume of water entering the well, dV = Adh

23. Recuperation Test – yield of an open well If Q is the rate of discharge in the well at the time t, under the well in time t hours is taken by dV = Qdt Q= Kh dV = Khdt Then, Khdt = -A dh Minus indicates that h decreases as time t increases.

24. Recuperation Test – yield of an open well Knowing the values of h1,h2 and T from recuperation test, the quantity K/A can be calculated. Q=KH Q=(K/A)AH

25. Water requirements for crops Principal crops and crops seasons in INDIA Crops classified based on Agricultural classification Classification based on crop season Classification based on irrigation requirement

26. Water requirements for crops Agricultural classification Field crops: wheat, rice, maize, barley, oats, great millet, sprinkled millet, gram, pulses etc. Commercial crops: sugar cane, cotton, tobacco, hemp, sugar beat etc. Oil seed crops: mustard, ground nut, sesame, linseed, caster etc. Horticulture crops: fruit crops, various vegetable and flower crops. Plantation crops: tea, coffee, cacao, coconut, rubber etc. Forage crops: fodder, grass etc. Miscellaneous crops: medicinal crops, aromatic crops, sericulture crops, condiments and spices.

27. Water requirements for crops Classification based on crop seasons Rabi crops or winter crops: these crops are sown in autumn (or October) and are harvested in spring (or March). Ex: Gram, wheat, peas, mustard, linseed, potato etc. Kharif crops or monsoon crops: these crops are sown by the beginning of the southwest monsoon and are harvested in autumn. Ex: rice, maize, sprinkled millet, great millet, pulses, ground nut etc. Perennial crops: these are the crops that require water throughout the year. Ex: sugar cane, fruits, vegetables. Eight months crops: these crops, such as cotton, require water for 8 months.

28. Water requirements for crops Classification based on irrigation requirements Dry crops : do not require water for irrigation, only rain water is sufficient for growth. Wet crops: cannot grow without water. Garden crops: these are the crops that require water throughout the year.

29. Water requirements for crops Crop ratio: it is the ratio of the area irrigated in the Rabi season to the area irrigated in Kharif season. Overlap allowance: the crop of some season may overlap some period of the next crop season, require water for the both the seasons simultaneously. Overlap allowance is the extra discharge for this purpose. Duty: represents the irrigating capacity of a unit water

30. Water requirements for crops Duty: represents the irrigating capacity of a unit water. It is the relationship between the area of the crop irrigated and the quantity of irrigation water required during the entire period of the growth of that crop. Ex: if 3 cumecs of water supply is required for a crop sown in an area of 5100 hectares, the duty of irrigation water will be Duty = 1700/3 = 1700 hectares/cumec

31. Water requirements for crops Delta: is the total depth of water required by a crop during the entire period of the crop is in the field and is denoted by the symbol Δ . Ex: if a crop requires a water about 12 waterings at an interval of 10 days, and a water depth of 10cm in every watering, then Delta = 12x10 = 120 cm = 1.2m If A is the area under that crop, the total quantity will be = 1.2 x A hectare-meters