Climate variability and change in the Altai-Dzungarian region and its hydrological impact

The Altai Mountains and also other mountainous areas of Central Asia experienced intermittent cold periods during the 20th-century warming of the Northern Hemisphere. Since the recent cold period in the 1980s, subsequent warm years in the Altai Mountains have accelerated the regional hydrological cycle. Previous studies found changes in snow cover duration, an acceleration of glacier recession, and permafrost degradation related to increasing soil temperatures. An improved knowledge of the long-term climatic variation and its drivers, and their hydrological impacts in the semi-arid mountainous Altai-Dzungarian region is required to better understand and predict impacts on regional agriculture and water availability. Climate observations from this rather inaccessible region are limited; thus, alternative datasets are used in this study. Based on tree-ring proxy data, firstly long-term climate variations were reconstructed back to the Little Ice Age followed by a statistical examination of teleconnections with large-scale atmospheric circulation patterns in order to reveal the driving forces of the regional climate. Secondly, weather station observations and interpolated APHRODITE precipitation and temperature data were used to analyze runoff changes in the Bulgan catchment in response to climate change from 1985 to 2012. Finally, GCM (CanESM2 and HadGEM2-AO) and statistically and dynamically downscaled RCM (SD_CanESM2 and RegCM4) simulations were evaluated and used in estimating future climate and hydrological change from 2030 to 2050 under the RCP4.5 and RCP8.5 scenarios. The hydrological impact of climate change on the predominantly snow-fed Bulgan catchment was assessed by conceptualizing the seasonal melt of glaciers and permafrost in the HBV-Light model. The analysis of tree-rings allowed for the reconstruction of long-term temperature (611 years) and precipitation time series (444 years) in the Altai-Dzungarian region. The results suggest that a cool and wet Little Ice Age was followed by warming in the 20th century, which was interrupted by short cool and wet periods caused by volcanic eruptions and intensified westerly and southwesterly winds connected with positive phases of the NAO, AO, and NINO3.4 variations. Since accumulated snow, summer rainfall and summer temperature significantly influence runoff variation in the Bulgan catchment, the +2.1°C temperature increase and 20% annual precipitation decrease from 1984 to 2015 resulted in a reduction of upstream river discharge in summer by 60%, but an increase in winter by 87%. Permafrost degradation might play a role in the increase in the winter baseflow since the glacierized area of the Bulgan catchment is only 0.13%. Compared to the reference period from 1985 to 2005, the temperature is projected to increase by +2.4°C (+2.9°C), and annual precipitation to increase by 13% (18%) under the RCP4.5 (RCP8.5) scenario from 2030 to 2050. Runoff is projected to increase in particular in spring (April and May) and even decrease in summer (June). An earlier snowmelt might make the intensified agriculture along the Bulgan River face water shortage during the growing season despite a projected precipitation increase in the 2040s

The history of rainfall data time-resolution in a wide variety of geographical areas

Collected rainfall records by gauges lead to key forcings in most hydrological studies. Depending on sensor type and recording systems, such data are characterized by different time-resolutions (or temporal aggregations), ta. We present an historical analysis of the time-evolution of ta based on a large database of rain gauge networks operative in many study areas. Globally, ta data were collected for 25,423 rain gauge stations across 32 geo graphic areas, with larger contributions from Australia, USA, Italy and Spain. For very old networks early re cordings were manual with coarse time-resolution, typically daily or sometimes monthly. With a few exceptions, mechanical recordings on paper rolls began in the first half of the 20th century, typically with ta of 1 h or 30 min. Digital registrations started only during the last three decades of the 20th century. This short period limits investigations that require long time-series of sub-daily rainfall data, e.g, analyses of the effects of climate change on short-duration (sub-hourly) heavy rainfall. In addition, in the areas with rainfall data characterized for many years by coarse time-resolutions, annual maximum rainfall depths of short duration can be potentially underestimated and their use would produce errors in the results of successive applications. Currently, only 50% of the stations provide useful data at any time-resolution, that practically means ta = 1 min. However, a sig nificant reduction of these issues can be obtained through the information content of the present database. Finally, we suggest an integration of the database by including additional rain gauge networks to enhance its usefulness particularly in a comparative analysis of the effects of climate change on extreme rainfalls of short duration available in different locations

The history of rainfall data time-resolution in a wide variety of geographical areas

Collected rainfall records by gauges lead to key forcings in most hydrological studies. Depending on sensor type and recording systems, such data are characterized by different time-resolutions (or temporal aggregations), ta. We present an historical analysis of the time-evolution of ta based on a large database of rain gauge networks operative in many study areas. Globally, ta data were collected for 25,423 rain gauge stations across 32 geographic areas, with larger contributions from Australia, USA, Italy and Spain. For very old networks early recordings were manual with coarse time-resolution, typically daily or sometimes monthly. With a few exceptions, mechanical recordings on paper rolls began in the first half of the 20th century, typically with ta of 1 h or 30 min. Digital registrations started only during the last three decades of the 20th century. This short period limits investigations that require long time-series of sub-daily rainfall data, e.g, analyses of the effects of climate change on short-duration (sub-hourly) heavy rainfall. In addition, in the areas with rainfall data characterized for many years by coarse time-resolutions, annual maximum rainfall depths of short duration can be potentially underestimated and their use would produce errors in the results of successive applications. Currently, only 50% of the stations provide useful data at any time-resolution, that practically means ta = 1 min. However, a significant reduction of these issues can be obtained through the information content of the present database. Finally, we suggest an integration of the database by including additional rain gauge networks to enhance its usefulness particularly in a comparative analysis of the effects of climate change on extreme rainfalls of short duration available in different locations