1. Applying TxRR to Texas Coastal Basins
Victoria Samuels
CE 394K.2

2. Background
Three state agencies have been assigned the cooperative task of determining just how much water will be needed to maintain our state's streams, rivers, Bays & Estuaries. Currently, the Texas Water Development Board is working with the Texas Parks and Wildlife Department and the Texas Natural Resource Conservation Commission (TNRCC) to collect data and conduct studies on the needs for instream flows and freshwater inflows to the estuarine systems

3. TxRR development Estimates runoffs from ungaged watersheds
Estimates streamflows on a daily basis
Analyzes soil moisture continuously
Analyzes direct runoffs by event

4. TxRR framework Agricultural Research Service (ARS)
12 monthly depletion factors rather than 1 annual factor (used in soil moisture)
Soil Conservation Service (SCS) Curve Number Method
Parameter Optimization using Newton’s Method
Changed to Genetic Algorithm
SCS Unit Hydrograph
Changed to Gamma function (GAMMADIST function in Excel)

5. TxRR Water Balance Precipitation P Initial Abstraction Ia
Direct Runoff QD
Infiltration F
Maximum Soil Moisture SMMAX =
Soil Moisture SM + Soil Retention S
Stream Flow
Base Flow QB
Percolation
(not modelled)

6. How Soil Moisture works - Time
Precipitation
Event
“i-1”
Precipitation
Event
“i”
Precipitation
Event
“i+1”
In “i-1” time:
SM SM1
Time between Precipitation Events = ti
In “i” time:
SM SM1

7. Sequence of Model Before Precipitation Event “i” It Rains! Pi at ti
Soil Moisture depleted from end of event “i-1” to before event “i”
Soil Retention increases
It Rains! Pi at ti
Initial abstractions subtracted
Runoff produced
Excess goes to Infiltration

8. Soil Moisture Depletion
SM2i = SM1i-1 exp (-amti)
SM2i soil moisture right before i precipitation
SM1i-1 soil moisture right after i-1 precipitation
am monthly depletion factor for month m
ti arrival time in days of the i precipitation
Monthly depletion factors
Arrival time of precipitation events
INPUT:

9. Soil Retention Si = SMMAX – SM2i SMMAX the maximum soil moisture
SM2i soil moisture right before i precipitation
Si soil retention at the beginning of i precipitation
Soil Retention is the dry space in the soil column, the room
available for additional moisture or an abstraction.
INPUT:
SMMAX optimized in model

10. Direct Runoff QDi = Pei2 / (Pei + Si) Pei = Pi – Iai Iai = abst1 * Si
QDi direct runoff from i precipitation
Pei effective precipitation
Iai initial abstraction from i precipitation
abst1 initial abstraction coefficient (usually 0.2)
Essentially the SCS Direct Runoff Equation
Pi precipitation from i event
abst1, user defined
INPUT:

11. New Soil Moisture SM1i = SM2i + Fi Fi = Pi – Iai - QDi
SM1i new soil moisture right after i precipitation
Fi infiltration after i precipitation

12. Coming Soon
Baseflow Calculations – modelled with Soil moisture conditions
Daily Streamflow Simulation
Applying the Model
Acknowledgements: Barney Austin, Dr. Maidment,
Richard Gu, Jerry Perales