# 1:4.2 Climate Change

The impact of global climate change on water supply is a major area of research. Climate change can be simulated with SWAT+ by manipulating the climatic input that is read into the model (precipitation, temperature, solar radiation, relative humidity, wind speed, potential evapotranspiration and weather generator parameters). A less time-consuming method is to set adjustment factors for the various climatic inputs.

SWAT+ will allow users to adjust precipitation, temperature, solar radiation, relative humidity, and carbon dioxide levels in each subbasin. The alteration of precipitation, temperature, solar radiation and relative humidity are straightforward:

$$R\_{day}=R\_{day}\*(1+\frac{adj\_{pcp}}{100})$$                                                                                                                                 1:4.2.1

where $$R\_{day}$$ is the precipitation falling in the subbasin on a given day (mm H$$*2$$O), and $$adj*{pcp}$$ is the % change in rainfall.

$$T\_{mx}=T\_{mx}+adj\_{tmp}$$                                                                                                                                           1:4.2.2

where $$T\_{mx}$$ is the daily maximum temperature (°C), and $$adj\_{tmp}$$ is the change in temperature (°C).

$$T\_{mn}=T\_{mn}+adj\_{tmp}$$                                                                                                                                          1:4.2.3

where $$T\_{mn}$$ is the daily minimum temperature (°C), and $$adj\_{tmp}$$ is the change in temperature (°C).

$$\overline T\_{av} =\overline T\_{av} +adj\_{tmp}$$                                                                                                                                            1:4.2.4

where $$\overline T\_{av}$$ is the daily mean temperature (°C), and $$adj\_{tmp}$$ is the change in temperature (°C).

$$H\_{day}= H\_{day}+ adj\_{rad}$$                                                                                                                                        1:4.2.5

where $$H\_{day}$$ is the daily solar radiation reaching the earth’s surface ($$MJ m^{-2}$$), and $$adj\_{rad}$$ is the change in radiation ($$MJ m^{-2} d^{-1}$$).

$$R\_h=R\_h +adj\_{hmd}$$                                                                                                                                              1:4.2.6

where $$R\_h$$ is the relative humidity for the day expressed as a fraction, and $$adj\_{hmd}$$ is the change in relative humidity expressed as a fraction.

SWAT+ allows the adjustment terms to vary from month to month so that the user is able to simulate seasonal changes in climatic conditions.        

Changes in carbon dioxide levels impact plant growth. As carbon dioxide levels increase, plant productivity increases and plant water requirements go down. The equations used to account for the impact of carbon dioxide levels on plant water requirements are reviewed in Chapters 2:2 and 5:2. When carbon dioxide climate change effects are being simulated, the Penman-Monteith equation must be used to calculate potential evapotranspiration. This method has been modified to account for $$CO\_2$$   impacts on potential evapotranspiration levels. 

Table 1:4-2: SWAT+ input variables that pertain to climate change.

| Description                                                                                                | Source Name | Input Name | Input File                                                                                  |
| ---------------------------------------------------------------------------------------------------------- | ----------- | ---------- | ------------------------------------------------------------------------------------------- |
| $$adj\_{pcp}$$: % change in rainfall for month                                                             | rfinc       |            | .sub                                                                                        |
| $$adj\_{tmp}$$: increase or decrease in temperature for month (°C)                                         | tmpinc      |            | .sub                                                                                        |
| $$adj\_{rad}$$: increase or decrease in solar radiation reaching earth’s surface for month (MJ m$$^{-2}$$) | radinc      |            | .sub                                                                                        |
| $$adj\_{hmd}$$: increase or decrease in relative humidity for month                                        | huminc      |            | .sub                                                                                        |
| $$CO\_2$$: carbon dioxide level at the start of the simulation (ppmv)                                      |             | co2        | [parameters.bsn](https://swatplus.gitbook.io/io-docs/introduction-1/basin-1/parameters.bsn) |
| Potential evapotranspiration method                                                                        | pet         | pet        | [codes.bsn](https://swatplus.gitbook.io/io-docs/introduction-1/basin-1/codes.bsn)           |


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