Hydro-Meteorological Trends in an Austrian Low-Mountain Catchment

While ongoing climate change is well documented, the impacts exhibit a substantial variability, both in direction and magnitude, visible even at regional and local scales. However, the knowledge of regional impacts is crucial for the design of mitigation and adaptation measures, particularly when changes in the hydrological cycle are concerned. In this paper, we present hydro-meteorological trends based on observations from a hydrological research basin in Eastern Austria between 1979 and 2019. The analyzed variables include air temperature, precipitation, and catchment runoff. Additionally, the number of wet days, trends for catchment evapotranspiration, and computed potential evapotranspiration were derived. Long-term trends were computed using a non-parametric Mann–Kendall test. The analysis shows that while mean annual temperatures were decreasing and annual temperature minima remained constant, annual maxima were rising. Long-term trends indicate a shift of precipitation to the summer, with minor variations observed for the remaining seasons and at an annual scale. Observed precipitation intensities mainly increased in spring and summer between 1979 and 2019. Catchment actual evapotranspiration, computed based on catchment precipitation and outflow, showed no significant trend for the observed time period, while potential evapotranspiration rates based on remote sensing data increased between 1981 and 2019.

However, the magnitude and impact direction of climate change observations and projections vary significantly at the global and regional scale [16,17]. To give some examples, a runoff decrease was observed for the Chinese Wuding basin [18] or the Three-River-Headwaters region [19] while an increase in runoff was reported for the Chinese Kaidu basin [20] and the North-Eastern USA [21].
Precipitation observations indicate minor global changes despite a large, compensating variability with a decrease observed in the subtropics, the Mediterranean [24], southern Asia and Africa and increases observed in North America, South America and Eurasia [11,25]. Furthermore, a seasonal shift of precipitation (e.g. [22,26]) and runoff has been reported (e.g. [13]).
Several studies report increasing evapotranspiration trends for most of the Northern hemisphere (e.g. [22,[27][28][29][30]) while China experienced decreasing evapotranspiration rates over the past 50 years [31]. Some of these studies confirm the trend that dry areas become drier and wet areas become wetter, while some contradict [25,32].
The validation of observations is one of the most important tasks during hydrological assessments as faulty data obviously provoke wrong analysis results and conclusions. At the same time, particularly the validation of precipitation measurements is very demanding due to the spatial and temporal variability of rainfall and its stochastic nature. An appropriate validation strategy depends on several factors, such as the spatial distribution of stations, the recording and analysis frequency or the type of measurement device. While there is no standardized procedure that is generally applicable, validation strategies commonly comprise the following steps: (i) identification of documented defects, (ii) device specific boundaries, (iii) climatological boundaries, (iv) temporal variability, (v) intra-stational validation, and (vi) inter-stational variability [33,34].
The literature shows that the impact of climate change is widely acknowledged. At the same time it is obvious that the impacts highly vary at a regional and even local scale. However, this knowledge is crucial to develop measures to mitigate and counteract hydrological climate change impacts. In this paper we present and analyse the hydrometeorological data from an hydrological research catchment in Styria, Eastern Austria, that is monitored since 1979. Analysed climate variables include precipitation, air temperature, river flow, and evapotranspiration.

Hydrological research catchment Pöllau
The hydrological research basin (HRB) Pöllau was established in 1978 [35,36] and is currently operated by the Institute of Urban Water Management and Landscape Water Engineering at Graz University of Technology in cooperation with the Department 14 of the Federal State Styria. The decision to establish an HRB in the Pöllau sub-basin was based on a number of reasons: (i) the confining arched mountain ridge allows a clear delineation of the catchment, (ii) the loamy soils are characterized by low storage capacities minimizing the influence of subsurface flow on catchment hydrology, and (iii) the climate of the catchment with heavy storm events in the summer and relatively dry winters is representative for the Eastern alpine foothills [37]. The catchment covers 58.3 km 2 and is located in Styria, Austria, about 60 km north-east of the city of Graz (Figure 1). The elevation of the catchment ranges from 398-1279 m and the catchment land-cover is dominated by forest (ca. 44.6%) and grass-and cropland (ca. 51.5%) with a low degree of impervious areas (ca. 1.3%) [38]. The land-cover changes in the catchment are minor since the start of the observations in 1979.
The catchment comprises two main sub-catchments that are monitored: (i) the subcatchment Saifenbach/Dürre Saifen covering 23 km 2 (monitored 1997-2005 and since 2018) and (ii) the sub-catchment Prätisbach covering 21 km 2 (monitored since 1980). Additionally, the discharge at the joint catchment outlet of the both sub-catchments is monitored since 1980. Characteristic catchment properties are given in Table 1.  The first precipitation measurement in the HRB Pöllau was installed in 1979 (1, see 2 Figure 1 and Table 2). During the following year (1980) additional five precipitation gauges  To exclude as much doubtful data as possible from the subsequent analysis the  The validation steps (i)-(vi) were applied for the rainfall and discharge observations.

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The comparison of daily precipitation observations after validation shows a good corre-  of the trend. This approach has been successfully used to assess climate developments in 38 numerous earlier studies (e.g. [43][44][45][46]) and was therefore applied in the current study.

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The long-term trend of the air temperatures was analyzed based on the mean an-  The long-term trends for the catchment discharge were analysed for the gauge A while 51 the remaining gauges were utilized for data validation only.

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As for the precipitation and the temperature the long-term flow trends were also 53 analysed at a seasonal scale to identify temporal shifts in the stream flow behaviour.

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The catchment water balance was computed based on the observed precipitation and     The trend of the summer minima is also not significant at 6.3C with the lowest recording in the observed temperature trends including statistical trend properties is given in Table 3.    statistical trend properties is given in Table 3.    trend properties is given in Table 3.

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Water balance.pdf