CHANGES IN THE WATER BALANCE OF BOSNIA AND HERZEGOVINA AS A RESULT OF CLIMATE CHANGE

The analysis of meteorological data from the period 1961–2014 show the rise in the mean annual temperature in the entire territory of Bosnia and Herzegovina. The changes are more pronounced in the central – hilly part of the country. The increase in annual air temperature ranges from 0.4 to 1.0°C per decade, whereas temperature increases during vegetation period were up to 1.2°C per decade. Additionally, increases in air temperature over the last fourteen years are even more pronounced. Changed distribution of precipitation, significant variations and the increasing soil moisture deficit during vegetation period (April – September) are also evident in Bosnia and Herzegovina. The increase in air temperature combined with changes in the distribution of precipitation has resulted in a change of evapotranspiration and annual water balance. The main objective of this study was to determine and compare the severity of changes in mean annual water balance components between different regions in Bosnia and Herzegovina. Monthly weather data from 26 weather stations in Bosnia and Herzegovina, for the time period of 50 years (1967 – 2016) were used to determine and analyze impact of climate change on the following water balance components: temperature, precipitation, reference evapotranspiration, actual evapotranspiration, total runoff, soil moisture deficit and amount of snow. The results indicate that climate change has a substantial effect on the all water balance components. Air temperature (0.21 - 0.7 o C per decade), reference evapotranspiration (0.61 - 42.81 mm per decade) and soil moisture deficit (1.35 - 27.71 mm per decade) show an increasing trend over the entire territory of Bosnia and Herzegovina with the strongest increase in the north-west part of the country.


INTRODUCTION
Based on the analysis of meteorological data from the period 1961-2016, the mean annual temperature is showing a continuous rise on the entire territory of Bosnia and Herzegovina (BiH).The increase in annual air temperature ranges 1 Sabrija Čadro (corresponding author: s.cadro@ppf.unsa.ba),Mirza Uzunović, Jasmnika žurovec, University of Sarajevo, Faculty of Agriculture and Food Science, BOSNIA AND HERZEGOVINA; Salwa CHERNI-ČADRO, Hydro-Engineering Institute Sarajevo (HEIS), BOSNIA AND HERZEGOVINA.Paper was presented at the 10 th CASEE Conference, Sarajevo 2019 Notes: The authors declare that they have no conflicts of interest.Authorship Form signed online.from 0.4 to 1.0°C, with more pronounced changes in the continental part (Radusin et al., 2016).There is a positive trend in the mean (0.2-0.5 o C per decade), the maximum (0.3-0.6 o C per decade) and the minimum temperatures (0.3-0.5 o C per decade) throughout the year (Popov et al., 2018b;Trbic et al., 2017), with a significant increase in the frequency of warm extremes (Popov et al., 2018a).
The average annual precipitation is 1,255 mm and it is characterized with the high variation in spatial distribution, which ranges between 706 mm to 3,259 mm (Drešković & Mirić, 2013).In the period 1961-2016 most of the territory of BiH is characterized by a slight increase in the amount of annual sum of precipitation (Popov et al., 2019;Radusin et al., 2016;Vucijak et al., 2014).However, due to the increased intensity and variability of precipitation as well as the increased share of heavy rains in the total amount of rainfall, there is the increased risk of flooding, landslides, hail and soil erosion especially in the north-eastern part of BiH (Radusin et al., 2016).Thus, the most vulnerable municipalities to climate change in BiH can be found in this area (Zurovec et al., 2017).
The changes in the air temperature and the amount of precipitation results in changes in evapotranspiration (Cadro et al., 2019;Cadro et al., 2017) and the values of different soil water balance elements (Giugliano et al., 2013).Understanding the spatial and temporal variability of soil water balance elements such as evapotranspiration, water surplus, runoff, soil moisture deficit is essential for many hydrological, agricultural and environmental models (Guler, 2014;Huntington, 2006), especially in assessing regional climate change scenarios and natural hazards (landslides, floods, droughts, wildfires, disease epidemics, insect/animal plagues).
Changes in the soil water balance in the territory of BiH are until now analyzed only for a specific area using a smaller number of weather stations.Increase of potential evapotranspiration (PET) and soil moisture deficit (SMD) was found for Banja Luka, Tuzla, Zenica and Mostar (Čadro et al., 2016).Increasing trends in reference evapotranspiration, runoff and soil moisture deficit and decreasing trend in the amount of the snow were found for the Sarajevo located in central part of BiH (Čadro et al., 2018;Miseckaite et al., 2018).
The main challenges when performing this kind of research in BiH is limited data availability, discontinuity of data records and low station density (Cadro et al., 2019) that combined with complex interactions between the nature of the climate parameters and topographical features (Mardikis et al., 2005) especially in areas with the complex terrain such as BiH, additionally makes it difficult to obtain precise results.For all these reasons, when analyzing soil water balance, it is necessary to include as many weather stations (WS) as possible.
Based on the above, the main objective of this study was a precise calculation of all the soil water balance elements in BiH and analysis of their changes in time as a result of climate change taken into consideration long period of time and as many weather stations as possible.

MATERIAL AND METHODS Study area and data availability
BiH is a country located in south-eastern Europe.It covers an area of around 51,209 km², that is equivalent to 0.5 % of the Europe.Based on the climate regionalization, temperate warm and humid climate has a dominant surface share (64.62 %), followed by Df -humid boreal (24.53 %), and Mediterranean climates (10.71 %) (Drešković & Mirić, 2013).The land is mainly hilly to mountainous, with 5% is lowlands, 24% hills, 42% mountains, and 29% of karst area (Čadro et al., 2012;Radusin et al., 2016;Žurovec et al., 2017).
Map 1. Location of 26 selected weather stations in BiH.
Twenty-six weather stations (WS), relatively regularly distributed thought BiH, were selected for this study (Table 1, Map 1).These WS are collecting all daily climate data required for evapotranspiration and soil water balance calculation and almost all have historical data records for a period of at least 30 years (360 months).Exception are Bosanska Gradiška (240), Trebinje (192), Srebrenica (216), Višegrad (228) and Mrkonjić Grad (264) that are included for a better spatial coverage.
Daily climatic data, including mean (T mean ), maximum (T max ) and minimum (T min ) air temperature, sum of precipitation (P), mean relative humidity (RH), wind speed (u) and sunshine hours (n) for the period 1967 -2016 (50 years) were collected and averaged over each month.Data were provided by the Federal Hydrometeorological Institute Sarajevo and the Republic Hydrometeorological Service of the Republic of Srpska.Basic location characteristics and number of months used for each location are shown in Table 1.

Reference evapotranspiration (ET o )
Reference evapotranspiration (ET 0 ) required for the soil water balance calculation, was calculated using standard FAO-PM equation, given by (Allen et al., 1998): where ET o is the reference evapotranspiration (mm day -1 ), R n the net radiation at the crop surface (MJ m -2 day -1 ), G the soil heat flux density (MJ m -2 day -1 ), T mean the mean daily air temperature at 2 m height ( o C), u 2 the wind speed at 2 m height (m s -1 ), e s the saturation vapor pressure, e a the actual vapor pressure, e se a the saturation vapor pressure deficit, Δ the slope of the vapor pressure curve (kPa o C -1 ) and ƴ is the psychrometric constant (kPa o C -1 ).
All necessary parameters required for calculation of ET o where computed following the procedure developed in FAO-56 (Allen et al., 1998) via REF-ET: reference Evapotranspiration Calculator (Allen & Zhenguli, 2016) software.
Since reflected solar radiation (R s ) is required for R n calculation and this parameter is not measured on WS in BiH, it was estimated from the measured sunshine hours data (The Campbell-Stokes sunshine recorder) with the Ångström (1924) equation: where R a is the extraterrestrial radiation (MJ m -2 day -1 ) calculated for each day of the year and for different latitudes, from the solar constant (G sc = 0.0820 MJ m -2 min -1 ), the solar declination (δ) and the time of the year (J) and then by selecting the R a for 15th day of each month converted to monthly values, n is the actual duration of sunshine (h), N is the maximum possible duration of sunshine or daylight hours (h), as is the regression constant, expressing the fraction of extraterrestrial radiation reaching the earth on overcast days (n = 0) and a s + b s is the fraction of extraterrestrial radiation reaching the earth on clear days (n = N).
In the absence of actual solar radiation (R s ) measurements, the values a s = 0.25 and b s = 0.5 were used as suggested by Allen et al. (1998).
For the four WS (Bosanska Gradiška, Bosanski Novi, Trebinje and Višegrad) where measured solar radiation data (R s ) or sunshine hours data were missing for a certain month, solar radiation was estimated using Hargreaves' formula (Hargreaves & Samani, 1985) (Eq.3), as suggested in Allen et al. (1998): where R a is extra-terrestrial radiation (MJ m-2 d-1), T max maximum air temperature (°C), T min minimum air temperature (°C), k Rs adjustment coefficient (°C -0.5 ).Value of k Rs = 0.16 was used for Trebinje, k Rs = 0.14 for Višegrad and Bosanski Novi and k Rs = 0.13 for Bosanska Gradiška as suggested by Cadro et al. (2019) and Čadro et al. (2017a).Actual vapor pressure (e a ) was derived from relative humidity data (Allen et al., 1998) as: where e a is actual vapor pressure (kPa), e°(T min ) saturation vapor pressure at daily minimum temperature (kPa), e°(T max ) saturation vapor pressure at daily maximum temperature (kPa), RH max maximum relative humidity (%), RH min minimum relative humidity (%).In the absence of relative humidity data, e a was estimated by assuming that the dew pint temperature (T dew ) is close to daily minimum temperature (T min ) (Allen et al., 1998).When wind speed was not available, the average regional wind speed value was used.

Soil water balance
Monthly water balance was calculated by using Thornthwaite-Mather method (Thornthwaite & Mather, 1955;Thornthwaite & Mather, 1957) that was modified and later described in Dingman (2002).Except data on monthly precipitation (P) and evapotranspiration (ET o ) applied water balance requires data on soil available water content (SOIL max ).The value SOIL max = 100 mm was used (McBean et al., 1995) since it is regionaly (BiH, Serbia and Croatia) the most commonly used value (Šimunić, 2013;Vlahinić, 2004).
To detect the trends within time series of water balance components (annual precipitation -P, reference evapotranspiration -ET 0 , actual evapotranspiration -AET, soil moisture deficit -SMD, total runoff -TRO and SNOW) parametric method of linear regression was used, as shown in following equation: where x is the explanatory variable, y the dependent variable, b the slope of the line and a the intercept.The slope (b) indicates the mean temporal change of the studied variable.Positive values of the slope show increasing trends, while negative values of the slope indicate decreasing trends (Gocic & Trajkovic, 2013, 2014).

RESULTS AND DISCUSSION
The descriptive statistics (mean and CV) and the slope (b) for all analyzed climate and soil water balance elements (T mean , P, ET 0 , AET, SMD, TRO and SNOW) at 26 selected WS for the period 1967 -2016, are summarized in table 2, 3 and 4. Note: b -slope ( o C year -1 and mm year -1 ), CV -Coefficient of variation (%).
The mean annual air temperature in BiH is 10.20 o C, ranging from 1.49 0 C (WS Bjelašnica) to 15.03 o C (WS Mostar).The T mean shows increasing trends at all WS, ranging from 0.021 o C year -1 (WS Bileća) to 0.070 o C year -1 (WS Gradačac).In general, the entire territory of BiH shows worming trend of 0.4 o C per decade (Table 2).In addition, increases in air temperature over the last ten years are even more pronounced (Radusin et al., 2016).Trend intensity differ between country regions.It is highest in the north (0.036-0.070 o C year -1 ) and lowest in the south -Mediterranean part of the country (0.021-0.037 o C year -1 ).The highest variations in annual temperature are determined for the 3 WS (Bijeljina, Gradačac and Prijedor), all located at the north of BiH.
Mean annual precipitation in BiH is 1,094 mm, ranging from 756 mm (WS Bijeljina) to 1,747 mm (WS Trebinje).Trends of the annual amount of precipitation (P) for majority of analyzed WS are increasing (0.507-10.093 mm year -1 ).However, there is few locations (Mostar, Trebinje, Bugojno, Mrkonjić Grad), especially in Hercegovina (south BiH), that are showing decreasing trend of precipitation, ranging up to -1.914 mm per year.Similarly, in other studies decrease in total precipitation and the number of days with precipitation occurrence was found, whereas the duration of dry periods is prolonged over the entire East Herzegovina region (Popov et al., 2019).
On the other hand, the highest increasing trend is found for mountain WS Bjelašnica, ranging 10.093 mm of precipitation per year, or a 100 mm over a period of 10 years.The highest variations in annual precipitation was found for WS in south (Trebinje) and WS at higher altitudes (Bjelašnica and Čemerono).Also, as confirmed with other studies, trends are not spatially and temporally coherent (Popov et al., 2017).For the most of WS changes in annual precipitation are not significant, they are more pronounced by seasons (Radusin et al., 2016), especially during the last decade (2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016), resulting in increased frequency of months with extreme precipitation (Popov et al., 2017), catastrophic floods landslides and soil erosion.
Based on 26 analyzed WS, the mean ET 0 for BiH is 780 mm, while AET that beside climate conditions depend on the soil moisture, is 140 mm lower, or 640 mm in average, the difference represents SMD (Table 3).In study that included 108 WS in BiH and similar methodology, mean annual SMD of 143 mm was found (Cadro et al., 2019), the small difference in SMD confirms the accuracy of the data obtained in this study.Similarly, to the T mean reference evapotranspiration (ET 0 ) shows increasing trends at all WS, ranging from 0.061 mm year -1 (WS Trebinje) to 4.281 mm year -1 (WS Višegrad).
In general, for the entire territory of BiH increasing trend of ET 0 is 20.59 mm per decade (Table 3).The highest trend is found for the area from Drvar in the west to Banja Luka at the north of the country, as well as the area around Višegrad in the east.Note: b -slope (mm year -1 ), CV -Coefficient of variation (%).
AET trend has a similar spatial distribution but higher variation than ET 0 trend.For the south, north-west and north-east part decreasing (from -0.025 to -1.900 mm year -1 ) or low increasing trend (0.028-0.597 mm year -1 ) was found.While for the central part, from Drvar to Višegrad, or the mountainous region of the country (Dinarides), high increasing trend were recorded (0.597-4.054 mm year -1 ).AET can be a measure of agricultural water productivity, these results indicate an improvement of general conditions for agricultural production in the central -mountainous region of BiH.Thus, Climate change may have a positive effect on the yield in this area (Čadro et al., 2018;Radusin et al., 2016), and in same time negative effect on the rest of the country (north and south).Such positive effect also found in similar studies, especially for the northern Europe (Jacob et al., 2018), while the rest of Europe, especially the Mediterranean region, will mostly be negatively affected (Behrens et al., 2010).Note: b -slope (mm year -1 ), CV -Coefficient of variation (%).
The mean TRO for BiH is 465 mm, the highest was found for the WS Čemerno (1190 mm) and the lowest, almost ten time smaller, for the WS Višegrad (149 mm).It is interesting to note that these locations are relatively close to each other (Map 1), and this confirms the statement of the complexity of natural conditions in BiH and the need for as many WS as possible.TRO trends are not spatially coherent.The high variations in trend, from decreasing (-2.153 mm year -1 ) to increasing (8.770 mm year -1 ) trends were found.In average for BiH there a low positive trend of TRO, which amounts 9.70 mm per decade.
The average amount of snow for BiH is 227 mm, and this climate element, comparing to all others, shows decreasing trend for all WS.Exception is mountain WS Bjelašnica (6.611 mm year -1 ).The average decreasing trend of snow for BiH is -11.94 mm per ten years and the highest decreasing trend was found for WS Čemerno (-35.45 mm per decade).Also, the highest variations were found for mountain WS, Čemerno, Bjelašnica and Ivan Sedlo.

CONCLUSIONS
Linear regression was applied to determine soil water balance response to climate change in BiH.The air temperature, precipitation, FAO-56 PM reference evapotranspiration and water balance components: actual evapotranspiration, total runoff, soil moisture deficit and amount of snow trends were analyzed.Monthly weather data from 26 weather stations, for the time period of 50 years (1967 -2016) were used.
If observed on the level of BiH, the results obtained show increasing trends in T, P, ET 0 , AET, SMD and TRO series and decreasing trend in the amount of the SNOW.However, at the regional level, there are a lot of differences between climate elements trends.This is especially evident for the P, AET and TRO.At the regional level, as the result of climate change, the highest increase in dry conditions was found for the north-west BiH (Bosanski Novi, Prijedor, Sanski Most, Banja Luka, Gradačac), where high increase in temperature (0.36 -0.70 o C per decade) and SMD (18.04 -27.39 mm per decade) trends were identified, while the trend of AET is showing decline for the most of weather stations.Similarly, high increase of SMD and decline of AET was found in the south, for the weather stations Mostar, 71 mm per decade), however, there the intensity of the temperature increase is lower (0.21 -0.37 o C per decade).
When it comes to surplus, the biggest changes or the highest increasing trends are in the area of Bjelašnica, Trebinje and Drina and Bosna river basins.Area already affected by heavy rains cosign soil erosion, landslides and floods (2010,2014,2019).In same time, increasing trends of AET are indicating an improvement of general conditions for agricultural production in the centralhilly region of BiH, from Drvar in the west to Višegrad in the east.
Based on these results, priority areas for the regionally specific climate change adaptation measures, protection from natural hazards (drought, floods, landslides) as well as actions for the disaster risk reduction could be identified.

Table 1 .
Location, observation periods and climate characteristics of 26 selected weather stations (WS) in BiH.

Table 2 :
Results for the statistical tests for the annual air temperature (T mean ) and sum of precipitation (P) -26 weather stations (WS) inBiH, period 1967BiH, period   -2016.  .

Table 4 .
Results for the statistical tests for the annual total runoff (TRO) and