I computed the daily reference evapotranspiration $ET_0$ using the Crop evapotranspiration - Guidelines for computing crop water requirements - FAO Irrigation and drainage paper 56. The outputs seems realistic regarding the dynamic and the annual total but the time series contains negative values (about 1% of the time series).
The computation involves many equations for the sub-terms but the general formula is the following:
$$ ET_0 = \frac{0.408 \Delta (R_n - G) + \gamma \frac{900}{T + 273}U_2(e_s - e_a)}{\Delta + \gamma(1+0.34 U_2)} $$
with
- $ET_0$ reference evapotranspiration [mm day-1],
- $R_n$ net radiation at the crop surface [MJ m-2 day-1],
- $G$ soil heat flux density [MJ m-2 day-1] (=0 in this case),
- $T$ mean daily air temperature at 2 m height [°C],
- $U_2$ wind speed at 2 m height [m s-1],
- $e_s$ saturation vapour pressure [kPa],
- $e_a$ actual vapour pressure [kPa],
- $e_s - e_a$ saturation vapour pressure deficit [kPa],
- $\Delta$ slope vapour pressure curve [kPa °C-1],
- $\gamma$ psychrometric constant [kPa °C-1].
It appears that the negative values are due to a negative values of the net radiation $R_n$. A negative value of $R_n$ makes sense but what about negative $ET_0$ ? How should I deal with these as I intend to use $ET_0$ as input for hydrological modelling ?