Page 88 - Ecuador's Banana Sector under Climate Change

ecuador’s banana sector under climate change: an economic and biophysical assessment to promote a sustainable and climate-compatible strategy
GDD = Days per month (Average Monthly Temperature - Base Temperature = 13oC)
If answer is negative, assume 0; if the mean monthly temperature exceeds 35oC, then the GDD were 0 due to high temperatures (Thomas et al., 1998; Turner and Lahav, 1983).
In addition, TDU for banana were estimated, due to the fact that other factors than temperature are influencing bunch initiation in bananas. In several studies, there was variation in the GDD necessary for bunch initiation for the different locations, soils and planting material. The relationship between TDU and GDD for a time period is:
TDU=GDD * Pf * Wf (oC d)
Where Pf is a scalar (0.0 to 1.0) for photoperiod and Wf is a scalar (0.0 to 1.0) for soil water balance. Pf was not taken into account in this study. The soil water balance (Wf) was estimated monthly, from the ratio of rainfall: potential evaporation (Rain: PET) taking Wf as 1.0 if the ratio fell between 1.0 and 1.1. If Rain: PET was above 1.1 then Wf = 1
+ 0.2(1– Rain: PET), allowing for a negative effect for excess of water (Fortescue et al., 2011).
2.2.2 Estimation of water deficit
The irrigation water need or water deficit for banana was estimated as
the difference between the crop water need as measured by actual evapotranspiration(AET) and that part of the rainfall which can be used by the crop (the effective rainfall –Erain-). It was determined on a monthly basis and then summarized for the year.
Water Deficit (Irrigation Water Need) = AET – ERain
2.2.3 Monthly potential evapotranspiration
Potential evapotranspiration (PET) was estimated on a global scale to calculate the TDU and the banana water deficit. PET is a measure of the ability of the atmosphere to remove water through evapotranspiration (ET) processes. PET has been defined as ET of a reference crop in optimal conditions, having the following characteristics: well watered grass with an assumed height of 12 cm, a fixed surface resistance of 70 seconds per metre (s/m) and an albedo of 0.23 (Allen et al., 1998). Other methods of calculating PET exist, however, and the Hargreaves model was chosen to model PET globally for this study (Hargreaves and Allen, 2003). This method performed almost as well as the FAO Penman-Monteith, but required less parameterization (Hargreaves and Allen, 2003; Trajkovic, 2007), allowing a finer resolution (at 10 km resolution).
The Hargreaves model uses mean monthly temperature (Tmean) and global solar radiation (Rs) at the surface to calculate PET, as shown below:
PET = 0.0135 * (Tmean + 17.8) * Rs *days per month
The Rs is expressed in units of water evaporation (mm = 2.45 W m-2)
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