Applying TxRR to Texas Coastal Basins

Victoria SamuelsCE 394K.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.

http://hyper20.twdb.state.tx.us/data/bays_estuaries/b&epage.html

TxRR development Estimates runoffs from ungaged

watersheds Estimates streamflows on a daily

basis Analyzes soil moisture

continuously Analyzes direct runoffs by event

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)

TxRR Water BalancePrecipitation P

Direct Runoff QD

Initial AbstractionIa

Infiltration F

Maximum Soil Moisture SMMAX =

Soil Moisture SM + Soil Retention S

Percolation (not modelled)

Base Flow QB

Stream Flow

How Soil Moisture works - Time

PrecipitationEvent“i-1”

PrecipitationEvent

“i”

PrecipitationEvent“i+1”

In “i-1” time:

In “i” time:

SM2 SM1

SM2 SM1

Time between Precipitation Events = ti

Sequence of Model Before Precipitation Event “i”

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

Soil Moisture Depletion

Monthly depletion factors Arrival time of precipitation events

SM2i = SM1i-1 exp (-mti)

SM2i soil moisture right before i precipitationSM1i-1 soil moisture right after i-1 precipitationm monthly depletion factor for month mti arrival time in days of the i precipitation

INPUT:

Soil Retention

SMMAX optimized in model

Si = SMMAX – SM2i

SMMAX the maximum soil moistureSM2i soil moisture right before i precipitationSi soil retention at the beginning of i precipitation

INPUT:

Soil Retention is the dry space in the soil column, the room available for additional moisture or an abstraction.

Direct RunoffQDi = Pei

2 / (Pei + Si)Pei = Pi – Iai

Iai = abst1 * SiQDi direct runoff from i precipitationPei effective precipitationIai initial abstraction from i precipitationabst1 initial abstraction coefficient (usually 0.2)

Essentially the SCS Direct Runoff Equation

INPUT: Pi precipitation from i event abst1, user defined

New Soil Moisture

SM1i = SM2i + Fi

Fi = Pi – Iai - QDi

SM1i new soil moisture right after i precipitationFi infiltration after i precipitation

Coming Soon Baseflow Calculations – modelled

with Soil moisture conditions Daily Streamflow Simulation Applying the Model

Acknowledgements: Barney Austin, Dr. Maidment, Richard Gu, Jerry Perales