Basic Concepts and Equations
The Hargreaves method (Hargreaves and Samani, 1985) is one of two temperature-based evapotranspiration methods included in HEC-HMS. The method is based on an empirical relationship where reference evapotranspiration was regressed with solar radiation and air temperature data. The regression was based on eight years of precision lysimeter observations for a grass reference crop in Davis, CA. The method has been validated for sites around the world (Hargreaves and Allen, 2003). The method is capable of capturing diurnal variation in potential evapotranspiration for simulation time steps less than 24 hours.
Potential evapotranspiration ETo is computed as (Hargreaves and Samani, 1985):
| 1) |
ET_o = K * RS * (T + 17.8) |
where K is a coefficient, RS is solar radiation, and T is mean daily temperature.
Hargreaves and Samani (1982) developed an equation for determining solar radiation from extraterrestrial radiation and the measured temperature range. Extraterrestrial radiation is the amount of solar energy that would be on a horizontal plane on the earth's surface if the earth was not surrounded by an atmosphere:
| 2) |
RS = K_R_S * R_a * \sqrt{T_m_a_x - T_m_i_n} |
where KRS is a coefficient, Ra is extraterrestrial radiation, and Tmax and Tmin are the daily maximum and minimum air temperature, respectively.
When the Hargreaves Evapotranspiration Method is used in combination with the Hargreaves Shortwave Radiation Method, the computed Hargreaves evapotranspiration form is equivalent to Hargreaves and Allen (2003) Eq. 8:
| 3) |
RS = K_R_S K R_a (T + 17.8) \sqrt{T_m_a_x - T_m_i_n} |
Required Parameters
The only parameter required to utilize this method within HEC-HMS is the coefficient [deg C-1]. In addition, air temperature must be specified as a meteorologic boundary condition.
A Note on Parameter Estimation
While HEC-HMS provides a default coefficient value of 0.0135 deg C-1, this value must be calibrated and validated.