Challenges in Quantifying Changes in the Global Water Cycle

Abstract

Understanding observed changes to the global water cycle is key to predicting future climate changes and their impacts. While many datasets document crucial variables such as precipitation, ocean salinity, runoff, and humidity, most are uncertain for determining long-term changes. In situ networks provide long time series over land, but are sparse in many regions, particularly the tropics. Satellite and reanalysis datasets provide global coverage, but their long-term stability is lacking. However, comparisons of changes among related variables can give insights into the robustness of observed changes. For example, ocean salinity, interpreted with an understanding of ocean processes, can help cross-validate precipitation. Observational evidence for human influences on the water cycle is emerging, but uncertainties resulting from internal variability and observational errors are too large to determine whether the observed and simulated changes are consistent. Improvements to the in situ and satellite observing networks that monitor the changing water cycle are required, yet continued data coverage is threatened by funding reductions. Uncertainty both in the role of anthropogenic aerosols and because of the large climate variability presently limits confidence in attribution of observed changes.

Altered Precipitation and Flow Patterns in the Dunajec River Basin

“…For June and August, median (negative) differences in Q are not significant (p = 0.0742 and 0.0195, respectively), but are consistent with observed significant decreases in P. A smaller, significant positive difference in Q for February, together with a less important (p = 0.0078) positive difference in Q for January can likely be associated with higher air temperatures in these winter months (resulting in less frozen ground and more thawing) rather than higher precipitation (see Table 3). 3 •s −1 ). Thus, the main changes relate to river flow in May and September, and are in agreement with the identified, significant changes in precipitation for these months.…”

“…This is particularly true of precipitation due to its large variability and intermittent character [2]. On the other hand, good management of water resources should be based on reliable information about predicted changes in water cycle on the basis of the changes already observed [3].…”

Dominance of climate warming effects on recent drying trends over wet monsoon regions

“…The background dryness over land varies as climate changes, but the major climate parameter driving dryness changes remains unclear in many regions (Sherwood and Fu, 2014;Hegerl et al, 2015). In previous assessments, precipitation (P ), the amount of water supply, is regarded as a key variable for understanding variations in dryness, particularly in humid regions such as Asian monsoon regions Kitoh et al, 2013;Liu and Allan, 2013).…”

Projecting water yield and ecosystem productivity across the United States by linking an ecohydrological model to WRF dynamically downscaled climate data

“…The Intergovernmental Panel on Climate Change (IPCC, 2014) concludes that global surface temperature has increased 0.85 • C during 1880-2012, and increased 0.78 • C during 2003-2012 when compared to 1850-1900. Additionally, extreme precipitation and droughts have increased (Tebaldi et al, 2006;Trenberth, 2011;Bony et al, 2013;Hegerl et al, 2014). The global climate is projected to continue to change over this century and beyond (IPCC, 2014).…”

“…Mounting evidence has suggested that climate and its change has played an important role in controlling the water cycle by changes in evaporation, transpiration, and runoff (McCabe, 2002;Hamlet et al, 2007;Syed et al, 2010;Wang and Hejazi, 2011;Chien et al, 2013;Hegerl et al, 2014;Huntington and Billmire, 2014;McCabe and Wolock, 2014;Sun et al, 2014). Also, climate can exert a dominant control on vegetation structural and phenological characteristics through variations in air temperature, precipitation, solar radiation, wind, and CO 2 concentration (Nemani et al, 2003;Harding et al, 2011;Wang et al, 2014;Zhang, F. et al, 2014;.…”