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Lecture 5

# Lecture 5-Water Balance & Ground Water.docx

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School
Department
Earth Sciences
Course
EARTHSC 2WW3
Professor
John Macclean
Semester
Spring

Description
Water Balance & Groundwater 02/12/2014 1. What Is Water Balance? 1. Interpreting a water balance graph 1. X-axis is the months of the year and y-axis is mm of water equivalent 2. DEF: both precipitation and soil water are used by vegetation, but there still is not enough water for vegetation to grow optimally 3. -DST: both precipitation and soil water are used by vegetation 4. +DST: water from precipitation alone is sufficient for vegetation, and the excess water becomes soil water 5. SURP: the soil is full of water and all excess water from precipitation becomes runoff 2. How Do You Calculate A Water Balance? 1. Based on the Thornthwaite approach to rational classification of climate 2. What are the two most widely measured climatic variables on earth? 1. Temperature and precipitation 1. WHY? You can derive a lot from these two variables 3. Thornthwaite Model 1. Uses temperature and precipitation as inputs for the creation of water balance for a site 2. Precipitation is the input 1. Rain, snow, etc. 3. Evapotranspiration is the output, calculated from temperature 4. Incorporates soil water storage, depletion and recharge 4. Water Balance Tables 1. Potential Evapotranspiration (PE) 1. Is the amount of evaporation and transpiration that would occur if water was not limiting 2. PE is a function of T 3. Convert temperature which is an energy term to the depth of water that temperature could evaporate 4. If T < 0, PE = 0 5. If PE is greatest in DJF – southern hemisphere 6. If PE is greatest in JJA – northern hemisphere 2. Precipitation (P) 1. Long term average data (about 20-30 years) 2. Consistent precipitation = close to a body of water 3. P-PE 1. Subtraction 2. Negative number means PE > P 1. Likely drawing the water from the soil 4. Accumulated Water Loss 1. Only calculated when PE > P and P-PE is negative 2. Must add the numbers all the way through (a cycle not a calendar year) 3. Running sums of the negative P-PE values, as an absolute value 5. Storage Value (ST) 1. Assume a capacity of 300 mm, can be different at different locations 2. Read ACCWL vales off the supplied figure to determine soil water storage 3. The ST must be recharged by using the excess water from P – PE 4. Use the graph or use your calculator only when you have an accumulated water loss 1. Y = 300e^-0.0034x where x is the ACC WL value 5. When you have a surplus, you add the last ST value to the excess P-PE value 6. As you have more and more water loss it is difficult to draw water from the soil 7. 300 is the default maximum; storage values are at 300 and remain at 300 until there is a negative P – PE value (ACC WL) 6. Change In Storage (Delta ST) 1. Depletion of soil water storage 2. Decreases over time because as the soil dries out it is more difficult to extract water from it 3. Delta ST = STm-1 – STm where m is the month 7. Actual Evapotranspiration (AE) 1. If P < PE, AE = P + delta ST 2. If P > PE, AE = PE 3. NOTE: AE can never be larger than PE 8. Water Deficit (D) 1. Occurs when P < PE, then D = PE – AE 9. Water Surplus (S) 1. S = P – PE, if P > PE and ST = 300 1. If it is less than 300 the water is being used to recharge the soil 10. Check if your water balance table works 1. Use the total columns only 2. P = S + D = PE J F M A M J J A S O N D TOTAL PE 0 0 0 30 72 111 135 122 84 48 15 0 617 P 63 57 66 67 73 63 81 67 61 62 67 64 791 P-PE 63 57 66 37 1 -48 -54 -55 -23 14 52 64 174 ACC WL 48 102 157 180 ST 300 300 300 300 300 254 212 178 164 178 230 294 Delta ST 46 42 34 14 AE 0 0 0 30 72 109 123 101 75 48 15 0 573 D 2 12 21 9 44 S 57 57 66 37 1 218 11. Draw lines 1. AE – often the same as PE 2. PE 3. P 12. Identify regions 1. Water deficit: between PE and AE 2. Water surplus: P is above PE and ST is equal to 300 so the soil does not need anymore recharging 3. Soil moisture utilization: above P but below AE 4. Soil moisture recharge: P is above the PE line and being used to reach an ST of 300 5. Rainfall utilization: area underneath AE, PE, and P; all rainfall in this area is being utilized directly 5. Data Interpretation 1. Hemisphere 1. How can we tell? 1. Warmer (more precipitation) in J, F, M = southern 2. Warmer (more precipitation) in J, A, S = northern 3. Consistent PE means we are near the equator 2. Growing Season 1. What months? 1. Whenever we have precipitation 3. Magnitude of S and D 1. Just a qualitative question 4. Timing of S and D 1. When is S and D relative to peak growing season? 5. Continentally / Maritime 1. What can the water table tell us? 1. Close to water = regulatory effect on precipitation J F M A M J J A S O N D TOTAL PE 103 97 82 61 45 31 30 34 34 43 59 90 749 P 81 80 81 99 129 140 147 119 101 101 91 81 1250 P- PE ACC WL ST Delt a ST AE D S Back To Groundwater 6. Groundwater Management 1. What is it? 2. Change in storage = (P-E-T-F-I)+G+L+A 1. The Artificial Abstraction can be positive or negative 3. Example: The Nubian Sandstone 1. Very old water (up to 50,0
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