Table 5 Calculation method of vegetation water demand.

Evapotranspiration

\(\:{ET}_{0}=\frac{0.408\varDelta\:({R}_{n}-G)+r\frac{900}{T+273}{u}_{2}({e}_{s}-{e}_{a})}{\varDelta\:+r(1+0.34{u}_{2})}\)

\(\:{ET}_{0}\) denotes the reference crop evapotranspiration (mm d⁻¹), \(\:\varDelta\:\) represents the slope of the saturation vapor pressure curve (kPa °C⁻¹), and \(\:G\) is the soil heat flux density (MJ m⁻² d⁻¹). The psychrometric constant is denoted by \(\:r\) (kPa °C⁻¹), and \(\:{u}_{2}\) refers to wind speed measured at a height of 2 m (m s⁻¹). \(\:{e}_{s}\) and \(\:{e}_{a}\) represent the saturation vapor pressure and actual vapor pressure, respectively (both in kPa), while \(\:{R}_{n}\) denotes the net radiation at the land surface (MJ m⁻² d⁻¹).

Vegetation coefficient

K_CA=0.55

K_CW=0.85

K_CG=0.60

K_CA. K_CW and K_CG represent the vegetation coefficients for cropland, forest land, and grassland, respectively.

Soil moisture limitation factor

\(\:{K}_{s}=\left\{\begin{array}{c}1,\theta\:>{\theta\:}_{c}\\\:\frac{\theta\:-{\theta\:}_{z}}{{\theta\:}_{c}-{\theta\:}_{z}},{\theta\:}_{z}\le\:\theta\:\le\:\\\:0,\theta\:<{\theta\:}_{z}\end{array}\right.{\theta\:}_{c}\)

In the equation, θ denotes the soil moisture content; θ_c is the critical soil moisture threshold; and θ_z refers to the wilting point of the soil.

Vegetation distribution area

A_PA=4472.31km²

A_PW=162.05km²

K_CG=896.69km²

A_PA, A_PW and A_PG correspond to the spatial distribution areas of cropland, forest land, and grassland, respectively.

Vegetation ecological water demand

\(\:ET={ET}_{0}\times\:{K}_{c}\times\:{K}_{s}\times\:{A}_{P}\times\:{10}^{-3}\)

\(\:ET\) represents the ecological water demand of vegetation (m³), \(\:{ET}_{0}\) is the reference crop evapotranspiration (mm), \(\:{K}_{c}\) is the vegetation coefficient, \(\:{K}_{s}\) is the soil moisture constraint coefficient, and \(\:{A}_{P}\) denotes the vegetation-covered area (m²).