Hydrologic Investigation of the Ka’a’awa Valley: An Attempt to Calculate Water Balance

1. Hydrologic Investigation of the Ka’a’awa Valley: An Attempt to Calculate Water Balance James Douglas CSULB NSF-REU: GRAM Field Program June 2013

2. Physical Characteristics/Hydrology • 1,742 Acres (2.7 sq. miles) • 1715 mm/year average precipitation • Characterized by extreme topography, formed through volcanic processes (Koolau Divide) • Interesting surface hydrology • Dry drainage channels • Wetland present on main channel at rear of watershed • Disappearing/reappearing flow

4. What is a Water Balance Anyway? • P = Q + E + S • Why do we care? • Allows for better understanding of hydrogeology • Climate Analysis • Management implications

5. What is a Water Balance Anyway? • What do we need? • Precipitation Data • Geology/Soils Data • Solar Radiation Data • Temperature Data • Topographical Information • Vegetation Data Discharge Recharge Evapotranspiration

6. First Attempt: Water Balance Toolbox • Developed by James Dyer, Ohio University • Created to calculate outputs from existing datasets • Designed In NE US, supposedly adaptable to differing conditions

8. First Attempt: Realizations • Scale issues pervasive • Issues calibrating model • Solar Radiation maps • PET adjustment coefficients • Model limited by ignorance of vegetative impacts • Outputs neither especially helpful or accurate

9. Switching Gears • Toolset abandoned in favor of new calculations • New strategy • Create flow accumulation model from 5m DEM • Collect discharge readings from stream • Compare theoretical and measured surface flow • Literature Review: Shade and Nichols • Water balance calculated for S, SE, and Windward Oahu • Used as a reference for expected ET and GW recharge

10. Collecting Discharge Measurements • Transect established at bus turnaround stream crossing • Chosen as best point for consolidated channel flow • Width: 2m • Velocity readings and depths taken every 25cm • Midpoint formula employed to estimate flow

12. Estimating Base Flow • Issue: Discharge reading gathered during elevated flow conditions • Solution: Velocity/Depth reading taken at same location earlier in week, used to normalize flow • Formula: Subtract 2 cm from depths, divide discharges by ½ • Final output: 21,750 cm3/sec (0.77 ft3/sec)

13. Spatial Analysis • Flow accumulation model created in ArcMap • Converted accumulation to area above sample point • Precipitation grid averaged to rainflow/month • Area multiplied by precipitation gives theoretical discharge

17. Calculation Results • Average monthly precipitation: 161.18 mm • Average daily precipitation: 5.4 mm • Area above sample point: 3,124,170 square meters • Theoretical discharge: PxA= 194,270 ccm/s (6.86 cfs) • Measured discharge: 21,750 ccm/s • % of theoretical accounted for by measured: 11.20% • Importance: 88.8% precipitation either ET or recharge

18. Pulling in the Literature • Ka’a’awa water balance according to Shade and Nichols: Ka’a’awa Valley

19. An Estimated Water Balance

20. Conclusions/Future Directions • Scale issues still present in available datasets • Possible data to collect for future study: • Precipitation data within the Ka’a’awa valley • Vegetation classifications- density and spatial variability • Temperature monitoring that reflects E-W variation • More accurate soils data without gaps in AWC • More discharge data including storm event response, and value obtained above and below flow disappearance

21. Data Used • Precipitation Data • Rainfall Atlas of Hawaii • Temperature Data • Iowa State Mesonet • Topography Data • IFSAR 5m DTM • Soils Data • SSURGO NRCS database • Water Budget • Water Budget and the Effects of Land-Use Changes on Ground-Water Recharge, Oahu, Hawaii 1996, Shade, Patricia J.; Nichols, William D. USGS Professional Paper: 1412-C