### 1. Introduction

_{2}and other gases in the atmosphere of active radiation. Temperature is expected to rise with the increase of CO

_{2}and other gases. 2) Based on climate models, where rainfall and temperature shows significant changes in the future. Increased CO

_{2}in the atmosphere and changes in forest cover is the main reason suggested for climate change. Changes in precipitation and temperature obviously affect the groundwater [4].

### 2. Material and Methods

### 2.1. Description of Study

^{2}and the long of main river around 15.50 km. For more details, the location of the research is presented in the Fig. 1.

### 2.2. Model Description

#### 2.2.1. Soil water balance model

*P*

*,*

_{DAS}*P*

*,*

_{HT}*P*

*,*

_{KC}*P*

*= total rain, rain forest, mixed farm and open land respectively;*

_{LT}*L*

*,*

_{DAS}*L*

*,*

_{HT}*L*

*,*

_{KC}*L*

*= total area, forest area, mix farm area and open land respectively.*

_{LT}##### (8)

$$ETo=\frac{0.408\mathrm{\Delta}Rn+\gamma \frac{900}{(T+273)}{U}_{2}({e}_{s}-{e}_{a})}{\mathrm{\Delta}+\gamma (1+0.34\hspace{0.17em}{U}_{2})}$$*ETa*) is divided into two parts:

##### (9)

$$1)\hspace{0.17em}\text{If\hspace{0.17em}}{T}_{PN}>ETo\hspace{0.17em}\text{then\hspace{0.17em}the\hspace{0.17em}}ETa=ETo;$$##### (10)

$$2)\hspace{0.17em}\text{If\hspace{0.17em}the\hspace{0.17em}}{T}_{PN}<ETo,\hspace{0.17em}\text{then\hspace{0.17em}the\hspace{0.17em}}ETa={T}_{PN}+\mathrm{\Delta}SM$$*T*

*with the monthly evapotranspiration*

_{PN}*ETo*,

*APWL*) is divided into two parts:

##### (12)

$$\begin{array}{l}1)\hspace{0.17em}\text{In\hspace{0.17em}the\hspace{0.17em}dry\hspace{0.17em}months\hspace{0.17em}or\hspace{0.17em}}{T}_{PN}<ETo,\hspace{0.17em}\text{is\hspace{0.17em}done\hspace{0.17em}by\hspace{0.17em}adding\hspace{0.17em}up}\\ \text{the\hspace{0.17em}value\hspace{0.17em}difference\hspace{0.17em}}({T}_{PN}-ETo)\hspace{0.17em}\text{each\hspace{0.17em}month\hspace{0.17em}with\hspace{0.17em}a\hspace{0.17em}value}\\ \text{of\hspace{0.17em}}({T}_{PN}-ETo)\hspace{0.17em}\text{the\hspace{0.17em}previous\hspace{0.17em}month.}\end{array}$$##### (13)

$$2)\hspace{0.17em}\text{In\hspace{0.17em}the\hspace{0.17em}wet\hspace{0.17em}months\hspace{0.17em}or\hspace{0.17em}}{T}_{PN}>ETo,\hspace{0.17em}\text{then\hspace{0.17em}the\hspace{0.17em}value\hspace{0.17em}of\hspace{0.17em}}APWL\hspace{0.17em}\text{is\hspace{0.17em}equal\hspace{0.17em}to\hspace{0.17em}zero}$$*SM*) is divided into two parts:

*T*

*>*

_{PN}*ETo*,

*SM*value for each month is equal to field capacity In the dry months or

*T*

*<*

_{PN}*ETo*,

*SM*value is calculated by the equation

*SM*)

##### (16)

$$\begin{array}{l}\text{Water\hspace{0.17em}surplus\hspace{0.17em}}(WS)\hspace{0.17em}\text{occurs\hspace{0.17em}in\hspace{0.17em}wet\hspace{0.17em}months\hspace{0.17em}}({T}_{PN}>ETo),\hspace{0.17em}\text{obtained}\\ \text{by\hspace{0.17em}If\hspace{0.17em}}SM<SMC,\hspace{0.17em}\text{then\hspace{0.17em}}WS=0\hspace{0.17em}\text{and\hspace{0.17em}if\hspace{0.17em}not\hspace{0.17em}then\hspace{0.17em}}WS=S\end{array}$$*Vn*) depends on the amount of water balance and soil conditions. The data required are:

The coefficient of infiltration (

*In*), the value 0.2 to 0.5Groundwater flow recession factor (

*k*), the value from 0.4 to 0.7

Watershed area (

*L*), forest area (_{DAS}*L*), area mixed farms (_{HT}*L*) and area open land (_{KC}*L*) calculated from topographic maps (km_{LT}^{2})Input factor rain correction (

*αP*) = 1.2 [9].Rain in the forest (

*P*), rain the mix farm (_{HT}*P*) and rain in open land (_{KC}*P*) calculated by Eq. (1)–(3)._{LT}Net of rain forest (

*P*_{NT}*HT*), net of mix farm (*P*_{NT}*KC*) and net of open land (*P*_{NT}*LT*) calculated by Eq. (4)–(6).Total rain net (

*T*) calculated by Eq. (7)._{PN}Input factor temperature correction (

*αT*) = −1.0°C [9] into the Eq. (8)Potential evapotranspiration (

*ETo*) calculated by Eq. (8) with program computer Cropwat 8 for windows.Difference between

*T*with the monthly potential evapotranspiration calculated by Eq. (11)._{PN}Accumulated potential water loss (

*APWL*) calculated by Eq. (12)–(13).*SM*calculated by Eq. (14).Change in soil moisture (Δ

*SM*) calculated by Eq. (15).*WS*calculated by Eq. (16).Infiltration (

*I*) calculated by Eq. (17).The volume of groundwater is calculated by the Eq. (18).

#### 2.2.2. Climate change

##### (21)

$$Z=\{\begin{array}{l}(S-1)/\sigma s\mathrm{.......}jika\mathrm{.....}S>\mathrm{...0}\hfill \\ \mathrm{0......................}jika\mathrm{.....}S=\mathrm{...0}\hfill \\ (S+1)/\sigma s\mathrm{.......}jika\mathrm{.....}S<\mathrm{....0}\hfill \end{array}$$*X*

*and*

_{j}*X*

*is the data value of the data “*

_{k}*j*” and “

*k*”,

*j*>

*k*.

*Q*is the slope and

*B*is a constant.

*Q*in Eq. (22), it first needs to be calculated slope for all data with the equation:

*j*>

*k*.

*n*” value “

*X*

*” in a time series, it is obtained as*

_{j}*N*=

*n(n–*1

*)*/2 slope estimation

*Q*

*. Sens slope estimate is the median of*

_{i}*N*values

*Q*

*.*

_{i}*N*value of

*Q*

*is ranked from small to large, with an estimated Sens is:*

_{i}##### (24)

$$\begin{array}{c}Q={Q}_{[(N+1)/2]}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\text{if\hspace{0.17em}}\hspace{0.17em}\hspace{0.17em}N\hspace{0.17em}\text{is\hspace{0.17em}odd\hspace{0.17em}or}\\ Q=0,5({Q}_{(N/2)}+{Q}_{((N+2)/2)})\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\text{if\hspace{0.17em}}\hspace{0.17em}\hspace{0.17em}N\hspace{0.17em}\text{is\hspace{0.17em}even}\end{array}$$*B*” in Eq. (22), the value of “

*n*” data from the difference (

*X*

_{i}*–Q*.

*ti*) is calculated. The median value is the estimated of “

*B*”.

### 2.3. Data Collection and Analysis

#### 2.3.1. Data collection and analysis for climate change

#### 2.3.2. Data collection and analysis for model

### 3. Results and Discussion

*αP*) and temperature correction factor (

*αT*) as input. Thus, before using this model needs to be analyzed in advance whether there is a climate change study site by using Mann-Kendall models. Furthermore, the projected changes are analyzed by the method of non-parametric Sen’s by Sutapa (2015) [9] there has been a climate change in Bangga watershed as mention in Table 2. Thus this model can be used SWB model calculation results are presented in Table 3.

*α*) used in the calculation of Mann-Kendall is 0.001; 0.01; 0.05 and 0.1. According to the table of normal standard “Z” the values are: Z

_{0,001}= 3.292; Z

_{0,01}= 2.576; Z

_{0,05}= 1.96; Z

_{0,1}= 1.645.

*α*) = 0.001; 0.01; 0.05 and 0.1 respectively. If there is no sign (blank) means a significant level (

*α*) of more than 0.1 or can be said to be insignificant [9].