UNIVERSITY OF KWAZULU-NATAL EXAMINATIONS: JUNE 2011

Transcription

1 EXAMINATIONS: MODULE AND CODE: HYDROLOGY 210 HYDR210 DURATION: 3 HOURS TOTAL MARKS: 100 Internal Examiner : Ms ML Warburton : Mr RP Kunz : Mr H Bulcock : Ms KT Chetty : Prof SA Lorentz : Mr MJC Horan External Examiner : Prof C Everson STUDENTS ARE REQUIRED IN THEIR OWN INTERESTS, TO WRITE LEGIBLY NOTES: This paper consists 7 pages of questions (including one Equation Sheet). Please see that you have them all. Answer all questions. Answer each Question in a Separate Answer Book. Calculators may be used. QUESTION 1 Precipitation [18 marks] a) A farm dam which has recently been built has been designed to withstand the 50 year return period rainfall event. Briefly explain what this means. (2) (d) For clouds to form, cloud condensation nuclei need to be present. List two examples of cloud condensation nuclei. (1) In the Western Cape, 70% of the extreme events experienced are a result of frontal rainfall. Using a sketch, explain the frontal precipitation mechanism. (4) Raingauge catch is less than true rainfall reaching the ground, and differs depending on the type of raingauge used. A standard South African raingauge, a Nipher shielded raingauge at the same height above ground, and a ground level raingauge are located in the same area. For the same rainfall event, the following three (3) rainfall measurements are recorded: 53.1 mm, 46.2 mm and 43.8 mm. List which rainfall measurements are most likely to belong to which raingauge, and JUSTIFY your answer. (4) (e) Using a sketch, explain the relationship between drop diameter and fall velocity. (3) (f) Describe a type of catchment or situation where the Isohyetal method would be useful to calculate areal rainfall and discuss what the potential drawbacks of the Isohyetal method are? (4) 1

2 QUESTION 2 Evaporation [20 marks] Which equation shows that plants are more prone to soil water stress on hot days and why is this the case? (3) What are the four common reference evaporation surfaces used to derive crop coefficients? (4) When a warm, dry wind comes into contact with a large dam, explain (with the aid of a sketch) how the (i) evaporation rate, and (ii) water vapour content change as the wind blows across the water surface. (4) (d) (e) Describe ideal soil- and weather-related conditions that could result in high soil water evaporation losses? Sketches may be used to support your answer. (5) (i) Explain why the transpiration rate measured for a plantation of tall pine trees is higher than that for a nearby field of short grass. (3) (ii) How does the Penman-Monteith equation account for this difference in transpiration rates? (1) QUESTION 3 Interception [10 marks] Describe, with the aid of a labelled sketch the canopy interception process. On the sketch, distinguish between climatic and vegetation factors that influence interception loss. (10) QUESTION 4 Runoff [15 marks] Explain two differences between Horton s and Hewlett s theories of stormflow generation? (4) Explain with the aid of diagrams, two locations of flow convergence. (4) Explain the following: (i) The expected impacts of overgrazing a veld, on runoff volume? (ii) The potential impacts of afforestation on total runoff volume in a catchment? (4) (d) Briefly describe a simple and a complex method of measuring velocity in a stream. (3) 2

3 QUESTION 5 Soil Water [15 marks] Derive an expression for particle density, P s, given a laboratory setup where you are provided with four identical flasks, some soil, some water and a mass balance (scale). Use annotated diagrams to explain your derivation. (5) Define the term capillary rise and derive an equation for estimating capillary rise. Use appropriate diagrams to support your derivation. (5) Aggregated and compacted soils have different water retention characteristic curves. Discuss this statement using diagrams to support your explanation. (3) (d) Explain the term Hysteresis and discuss any one cause of the Hysteresis effect. (2) QUESTION 6 Groundwater [15 marks] Consider the unconfined and confined aquifers sketched in Figure 1. Boreholes A and C have been established in the confined aquifer. They are slotted over the lower section in the confined aquifer and sealed above the confined aquifer. Boreholes B and D have been established in the unconfined aquifer and are slotted near the base as shown. (i) Indicate on the sketch, the ground surface where the confined aquifer receives recharge. Indicate the ground surface where the unconfined aquifer receives recharge. (2) (ii) Indicate approximate water levels in boreholes A and C and the exact water levels in B and D. (3) (iii) If pumping commences from borehole C, what will happen to the water levels in A, B and D? (2) 3

4 PLEASE HAND IN THIS PAGE STUDENT NUMBER... A B C D vadose zone confined aquifer unconfined aquifer Distribution of water flow direction content above the phreatic surface at impervious bedrock equilibrium confining stratum Figure 1 4

5 The water content distribution above an aquifer is shown for an initial water table elevation and a final water table elevation in the Figure 2 below, where the final water table elevation is Z below the initial water table elevation. θ s is the saturated water content and θ r is the residual water content. V a represents the volume of water drained from the unsaturated zone after the water table drawdown. Using these terms and Z, give an expression for the: (i) Specific yield (1) (ii) Apparent specific yield (3) Ground surface V a Initial water table Z Final water table 0 θ r Water content θ s Figure 2 5

6 A homogenous aquifer drains into a straight section of stream as shown in Figure 3 below. Explain the physical meaning of each of the terms in the expression for the discharge into the stream: Q/w = K(h 2 -h o 2 )/(2L) borehole unconfined aquifer stream datum Figure 3 (4) QUESTION 7 Water Budget [7 marks] If a small scale irrigation farmer was to look at the monthly water budget for the farm area and finds the following: For February, a surplus of rainfall (P n ) over maximum evaporation. For April, a deficit of rainfall (P n ) as compared to maximum evaporation. (i) Explain in detail what, if anything, the farmer could conclude about the need to irrigate or not irrigate in those two months. (7) 6

7 EQUATION SHEET δ = 0.409cos[0.017(173 D)] h = arccos(-tanφ.tanδ) r = cos[0.017(186 D)] 1 degree = π/180 = radians E r = A Ta T DT / Ahi N d E r = /γrn + E /γ +1 a A T = A hi A hi A hi 3 E r = 10(0.142T )(T ) K cm D BC / N d VPD = e a e d RH = 100e d /e a K cm = E m /E r L = T E a = * T/( T) ( u)(100 RH)e a e a = * 10 R sw = R a (1 α)( n/N) f = ψ / E s cr l r R a = (h.sinφ.sinδ + cosφ.cosδ.sinh)/r 2 R lw = [εσt K 4 ][ (e a RH) 0.5 ][ n/N] N = 24.h / π = 4098e a /( T) 2 γ = 0.665*10-3 * P Z P = ρ = 1.292[273.2/( T)] E r = [1/( γ/ )] * [453R c (T Z) / (84 φ) 72R c + 3.6u 2 (T T d )] 7