Implementation of FAO Irrigation and Drainage Paper No. 56 (Allen et al., 1998) equations for adjusting tabulated mid-season (Kc_mid) and late-season (Kc_end) crop coefficients to local climatic conditions (wind speed, minimum relative humidity, and crop height). Adjust Crop Coefficients for Local Climate (FAO-56 Eq. 62 & 65)
Arguments
- kc_mid
Numeric vector. Tabulated mid-season crop coefficient (from FAO-56 Table 12).
- kc_end
Numeric vector. Tabulated late-season crop coefficient (optional).
- u2
Numeric vector. Mean daily wind speed at 2 m height during the stage (m/s). Valid range in FAO-56: 1.0 to 6.0 m/s.
- rh_min
Numeric vector. Mean daily minimum relative humidity during the stage (%). Valid range in FAO-56: 20% to 80%.
- h
Numeric vector. Mean plant height during the stage (m). Valid range in FAO-56: 0.1 to 10.0 m. Default is 1.0 m.
- clamp_inputs
Logical. If
TRUE(default), clamps inputs to the FAO-56 physical validity boundaries: \(1.0 \le u_2 \le 6.0\), \(20 \le RH_{min} \le 80\), and \(0.1 \le h \le 10.0\).
Value
A data.frame with:
- kc_mid_tab
Original tabulated Kc_mid
- kc_mid_adj
Climatically adjusted Kc_mid
- kc_end_tab
Original tabulated Kc_end (if provided)
- kc_end_adj
Climatically adjusted Kc_end (if provided)
- delta_kc
The computed climate adjustment offset
Details
Adjusts tabulated kc_mid and kc_end values from FAO-56 Table 12 to account
for local climatic conditions where minimum relative humidity differs from 45%
or wind speed at 2 m differs from 2.0 m/s.
The tabulated values for Kc_mid and Kc_end in FAO-56 represent a sub-humid climate with average daytime minimum relative humidity (RHmin) approx. 45% and calm to moderate wind speeds (u2) approx. 2.0 m/s.
For other conditions, FAO-56 specifies:
Mid-season adjustment (Eq. 62): $$Kc_{mid} = Kc_{mid(Tab)} + [0.04(u_2 - 2) - 0.004(RH_{min} - 45)] \left(\frac{h}{3}\right)^{0.3}$$
Late-season adjustment (Eq. 65): $$Kc_{end} = Kc_{end(Tab)} + [0.04(u_2 - 2) - 0.004(RH_{min} - 45)] \left(\frac{h}{3}\right)^{0.3}$$ (Applied only when \(Kc_{end(Tab)} > 0.45\). If \(Kc_{end(Tab)} \le 0.45\), no adjustment is made).
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration: Guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper No. 56, Rome, Italy. Equations 62 & 65.
Examples
# FAO-56 Example 26: Arid, windy climate
# u2 = 3.2 m/s, RHmin = 30%, h = 2.5 m, Kc_mid(Tab) = 1.20
wapor_adjust_kc_climate(kc_mid = 1.20, u2 = 3.2, rh_min = 30, h = 2.5)
#> kc_mid_tab kc_mid_adj delta_kc
#> 1 1.2 1.302 0.1023
# FAO-56 Example 27: Humid, calm climate
# u2 = 1.5 m/s, RHmin = 60%, h = 2.0 m, Kc_mid(Tab) = 1.20
wapor_adjust_kc_climate(kc_mid = 1.20, u2 = 1.5, rh_min = 60, h = 2.0)
#> kc_mid_tab kc_mid_adj delta_kc
#> 1 1.2 1.129 -0.0708
# With Kc_end (> 0.45 gets adjusted; <= 0.45 remains unadjusted)
wapor_adjust_kc_climate(kc_mid = 1.15, kc_end = 0.60, u2 = 4.0, rh_min = 25, h = 1.2)
#> kc_mid_tab kc_mid_adj delta_kc kc_end_tab kc_end_adj
#> 1 1.15 1.272 0.1215 0.6 0.722
wapor_adjust_kc_climate(kc_mid = 1.15, kc_end = 0.30, u2 = 4.0, rh_min = 25, h = 1.2)
#> kc_mid_tab kc_mid_adj delta_kc kc_end_tab kc_end_adj
#> 1 1.15 1.272 0.1215 0.3 0.3