jh_coef_hru

PRMS Parameter Name(s): JH_Coef_HRU_try

BAGIS-P Model Toolbox: bagis_p_climate_methods.tbx

BAGIS-P Model Name: JH_Coef

Required DATABIN Layer(s):

• • db_jul_tmin_grid: July_Min_Temp

  • db_jul_tmax_grid: July_Max_Temp

  • db_aug_tmax_grid: August_Max_Temp

  • db_aug_tmin_grid: August_Min_Temp

Field Name(s):

• JH_Coef_HRU_try

Parameter Description: This is a revised version of the algorithm from the Weasel. It uses the same formula for the JH Coefficient, which is:

JH_coef_hru = (27.5–0.25*(jh2 – jh1) – (jhelv /1000)),

where jh1 and jh2 are the minimum and maximum saturation vapor pressures, respectively, and jhelv is defined as the median elevation, expressed in feet, for the area for which the Jenson-Haise coefficient is being derived. The median elevation for each zone is automatically calculated by the method during determination of the Jenson-Haise coefficient.

Saturation vapor pressure is calculated by:

jhi = 6.1078 * exp((17.269 * Ti) / (Ti + 237.3)),

where jhi refers to either jh1 or jh2. When calculating jh1, Ti is the mean minimum temperature (degrees Celsius) for the warmest month of the year. When calculating jh2, Ti is the mean maximum temperature (degrees Celsius) for the warmest month of the year.

This is calculated using four 30-year avg. climate grids, with units of Celcius: august tmax, august tmin, july tmax, and july tmin. The model checks to see which of these two months are hotter by first averaging the tmax and tmin for each month, then finding the avg. value for the AOI. Only July and August are checked, because the warmest month of the year is always one of these two in the Northern Hemisphere.

Methods:

• 1. Use Map Algebra to calculate the MEAN July and August temperatures.

1. Find the average July and August temperatures in the AOI using Zonal Statistics as Table. Get MEAN July and August Field Value temperatures from the table.

2. Perform “Is_August_Warmer” script to control model branching based on whether July or August is warmer on average. If July is warmer, calculate the average July tmin and tmax for the AOI using Zonal Statistics as Table. If August is warmer, calculate the average August tmin and tmax for the AOI using Zonal Statistics as Table. Assign each MEAN to its respective field.

3. Merge Branches to calculate the hottest month average tmin(using average July tmin and August tmin for AOI value) and hottest month avg tmax (using average July tmax and August tmax for AOI value).

4. Convert to Celsius if needed using Calculate Value.

5. Calculate the minimum and maximum saturation vapor pressure (JH2) using the following expressions:

   1. • 6.1078 * math.exp((17.269 * converted hottest month avg tmin) / (converted hottest month avg tmin + 237.3))

      • 6.1078 * math.exp((17.269 * converted hottest month avg tmax) / (converted hottest month avg tmax + 237.3)

6. Make an integer DEM layer using Int. Calculate the MEDIAN elevation for each HRU using Zonal Statistics as Table. Join the MEDIAN value to the parameter table using Join Field and convert units from input elevation units to feet.

7. Add “JH_Coef_HRU” field to the parameter table. Calculate Field using the following expression:

   1. • (27.5 - 0.25*(%JH2% - %JH1%) - ([MEDIAN] /1000))

8. Delete joined median field.

9. Update parameter value for “fld_JH_Coef_HRU” on the parameter table.

BAGIS-P GUI Example: