1:1.2.5 Daily Net Radiation

Net radiation requires the determination of both incoming and reflected short-wave radiation and net long-wave or thermal radiation. Expressing net radiation in terms of the net short-wave and long-wave components gives:

$H_{net}=H_{day}\downarrow-\alpha*H_{day}\uparrow+H_L\downarrow-H_L\uparrow$ 1:1.2.11

or

$H_{net} = (1-\alpha) H_{day} + H_b$ 1:1.2.12

where $H_{net}$ is the net radiation ($MJ m^{-2} d^{-1}$), $H_{day}$ is the short-wave solar radiation reaching the ground ($MJ m^{-2} d^{-1}$), is the short-wave reflectance or albedo, $H_L$ is the long-wave radiation ($MJ m^{-2} d^{-1}$), $H_b$ is the net incoming long-wave radiation ($MJ m^{-2} d^{-1}$) and the arrows indicate the direction of the radiation flux.

1:1.2.5.1 Net Short-Wave Radiation

Net short-wave radiation is defined as $(1-\alpha) H_{day}$. SWAT+ calculates a daily value for albedo as a function of the soil type, plant cover, and snow cover. When the snow water equivalent is greater than 0.5 mm,

$\alpha=0.8$ 1:1.2.13

When the snow water equivalent is less than 0.5 mm and no plants are growing in the HRU,

$\alpha=\alpha_{soil}$ 1:1.2.14

where $\alpha_{soil}$ is the soil albedo. When plants are growing and the snow water equivalent is less than 0.5 mm,

$\alpha=\alpha_{plant} (1-cov_{sol})+\alpha_{soil} cov_{sol}$ 1:1.2.15

where $\alpha_{plant}$ is the plant albedo (set at 0.23), and $cov_{sol}$ is the soil cover index. The soil cover index is calculated

$cov_{sol}=exp(-5.0X10^{-5}*CV)$ 1:1.2.16

where $CV$ is the aboveground biomass and residue ($kg ha^{-1}$).

1:1.2.5.2 Net Long-Wave Radiation

Long-wave radiation is emitted from an object according to the radiation law:

$H_R=\varepsilon \sigma T_K^{4}$ 1:1.2.17

where $H_R$ is the radiant energy ($MJ m^{-2} d^{-1})$, $\varepsilon$ is the emissivity, $\sigma$ is the Stefan-Boltzmann constant ($4.903 10^{-9} MJ m^{-2} K^{-4} d^{-1})$, and $T_K$ is the mean air temperature in Kelvin (273.15 + $\degree C$). Net long-wave radiation is calculated using a modified form of equation 1:1.2.17 (Jensen et al., 1990):

$H_b=f_{cld} (\varepsilon_a -\varepsilon_{vs}) \sigma T_K^{4}$ 1:1.2.18

where $H_b$ is the net long-wave radiation ($MJ m^{-2} d^{-1}$), $f_{cld}$ is a factor to adjust for cloud cover, $\varepsilon_a$ is the atmospheric emittance, and $\varepsilon_{vs}$ is the vegetative or soil emittance.

Wright and Jensen (1972) developed the following expression for the cloud cover adjustment factor, $f_{cld}$:

$f_{cld}=a \frac{H_{day}}{H_{MX}}-b$ 1:1.2.19

where $a$ and $b$ are constants, $H_{day}$ is the solar radiation reaching the ground surface on a given day ($MJ m^{-2}d^{-1}$), and $H_{MX}$ is the maximum possible solar radiation to reach the ground surface on a given day ($MJ m^{-2}d^{-1}$).

The two emittances in equation 1:1.2.18 may be combined into a single term, the net emittance $\varepsilon'$. The net emittance is calculated using an equation developed by Brunt (1932):

$\varepsilon'=\varepsilon_a-\varepsilon_{vs}=-(a_1+b_1 \sqrt{(e)})$ 1:1.2.20

where $a_1$ and $b_1$ are constants and $e$ is the vapor pressure on a given day ($kPa$). The calculation of $e$ is given in Chapter 1:2. Combining equations 1:1.2.18, 1:1.2.19, and 1:1.2.20 results in a general equation for net long-wave radiation:

$H_b=-[a \frac{H_{day}}{H_{MX}}-b] [a_1+b_1 \sqrt{(e)}] \sigma T_k^4$ 1:1.2.21

Experimental values for the coefficients $a,b,a_1$, and $b_1$ are presented in Table 1:1.3. The default equation in SWAT+ uses coefficient values proposed by Doorenbos and Pruitt (1977):

$H_b=-[0.9 \frac{H_{day}}{H_{MX}}+0.1] [0.34-0.139\sqrt{(e)}] \sigma T_k^4$ 1:1.2.22

Table 1:1-3: Experimental coefficients for net long-wave radiation equations (from Jensen et al., 1990).

Region	(a,	b)	(a1,	b1)
Davis, California	(1.35,	-0.35)	(0.35,	-0.145)
Southern Idaho	(1.22,	-0.18)	(0.325,	-0.139)
England	not available	not available	(0.47,	-0.206)
England	not available	not available	(0.44,	-0.253)
Australia	not available	not available	(0.35,	-0.133)
General	(1.2	-0.2)	(0.39,	-0.158)
General-humid areas	(1.0	0.0)
General-semihumid areas	(1.1	-0.1)

Table 1:1-4: SWAT+ input variables used in net radiation calculations.

Definition	Source Name	Input Name	Input File
$\alpha_{soil}$: moist soil albedo	alb	alb	soils.sol
$T_{mx}$: Daily maximum temperature ($\degree C$)	max temp	tmpmax	.tmp
$T_{mn}$: Daily minimum temperature ($\degree C$)	min temp	tmpmin	.tmp
$H_{day}$: Daily solar radiation reaching the earth’s surface ($MJ m^{-2}d^{-1}$)	solrad	slr	.slr