# Temporal and Spatial Diversity of Renewable Groundwater Resources in the River Valley
## 3 Research Methods  
### Observation Period, Research Variants  
The quantitative assessment of the renewable groundwater resources can be identified with infiltration of recharge, so there is also a need to recognize the possibility of climate changes through the changes in a precipitation causing time-variable infiltration. Based on precipitation measurements in station located in the central part of the study area, the annual classification has been done in the reference to the average rainfall sum in 1999-2013 period [50]. The analysis concerning amount of deviation of monthly precipitation from average monthly precipitation sum also has been done (Figure 4). The amount of significant deviation of monthly precipitation in the range of 30-50% and over 50% was correlated with annual groundwater depth (GD) to define strength of this relationship (Table 1). Figure 4: Scheme of groundwater recharge dependency in shallow hydrogeological systems, precipitation analysis. A general trend line for annual precipitation could not be found. Every two or three years, there has been noted an average type of year due to precipitation sum. There also have been registered two dry and very dry years, as well as three wet and one very wet year. Precipitation analysis revealed the significant monthly deviation of precipitation and its amount per year. Deviation over 50% occurs with a frequency two to six times per year, while deviation in a range 30-50% occurs one to five times per year. The year 2009 was an exception, classified as normal with no significant deviation. In the time of 15 years of measurements, the largest deviations were positive values: 183% in 2006 and 171% in 2011, which can be interpreted as a short and intensive precipitation impulse, which usually does not transform into higher infiltration. Negative values reached \(-\)89% in 2000 and in 2011. There is a mosaic set in both cases, which means that precipitation changes and future infiltration changes cannot be predicted or forecast. Correlation of amount of deviation over 50% with annual groundwater depth is only notable in the presence of negative deviation (\(-\)0.36) and the direct correlation of annual precipitation sum with annual GD is very low (\(-\)0.19). Due to the lack of a clear relationship between precipitation and GD, it was decided to characterize renewable resource changes by analyzing groundwater levels and selecting extreme levels noted in1999-2013 period. Groundwater and surface water levels were measured in the groundwater and surface water monitoring network. The analysis is based on observations from the years 1999-2013, conducted in 56 piezometers and 21 water gauges (Figure 1). Basic statistical parameters, such as average, maximum, minimum, standard deviation, 1 st and 3 rd quartile and amplitude have been determined on a seasonal, annual and long-term basis. The statistics obtained at each monitoring point were transposed onto the entire study area using a geostatistical analysis, in this case: an ordinary kriging tool [48]. Seasonal groundwater level changes occurred in a similar way at all observation points throughout the 1999-2013 period, and also during the growing season, when the increasing GD is the dominant trend. Over the period of 1999-2013, the highest groundwater levels were recorded in 2011 at all observation points. At 12 of them, the levels periodically rose above the ground surface; at 3 points, it lasted 42 to 56 weeks [40]. In relation to precipitation, groundwater level in 2011 appeared to be the result of the cumulative impact of the positive deviations of precipitation in 2010 (very wet year) and 2011 (average year). The analysis made it possible to indicate the periods of risk and occurrence of hydrogeological drought [47]. The risk of hydrogeological drought was defined as a period in which the groundwater table level remained lower than the annual low average level (SNG).