TY - JOUR A1 - Brunello, Camilla Francesca A1 - Andermann, Christoff A1 - Marc, Odin A1 - Schneider, Katharina A. A1 - Comiti, Francesco A1 - Achleitner, Stefan A1 - Hovius, Niels T1 - Annually resolved monsoon onset and withdrawal dates across the Himalayas derived from local precipitation statistics JF - Geophysical research letters N2 - A local and flexible definition of the monsoon season based on hydrological evidence is important for the understanding and management of Himalayan water resources. Here, we present an objective statistical method to retrieve seasonal hydrometeorological transitions. Applied to daily rainfall data (1951-2015), this method shows an average longitudinal delay of similar to 15 days, with later monsoon onset and earlier withdrawal in the western Himalaya, consistent with the continental progression of wet air masses. This delay leads to seasons of different length along the Himalaya and biased precipitation amounts when using uniform calendric monsoon boundaries. In the Central Himalaya annual precipitation has increased, due primarily to an increase of premonsoon precipitation. These findings highlight issues associated with a static definition of monsoon boundaries and call for a deeper understanding of nonmonsoonal precipitation over the Himalayan water tower.
Plain Language Summary Precipitation in the Himalayas determines water availability for the Indian foreland with large socioeconomic implications. Despite its importance, spatial and temporal patterns of precipitation are poorly understood. Here, we estimate the long-term average and trends of seasonal precipitation at the scale of individual catchments draining the Himalayas. We apply a statistical method to detect the timing of hydrometeorological seasons from local precipitation measurements, focusing on monsoon onset and withdrawal. We identify longitudinal and latitudinal delays, resulting in seasons of different length along and across the Himalayas. These spatial patterns and the annual variability of the monsoon boundaries mean that oft-used, fixed calendric dates, for example, 1 June to 30 September, may be inadequate for retrieving monsoon rainfall totals. Moreover, we find that, despite its prominent contribution to annual rainfall totals, the Indian summer monsoon cannot explain the increase of the annual precipitation over the Central Himalayas. Instead, this appears to be mostly driven by changes in premonsoon and winter rainfall. So far, little attention has been paid to premonsoon precipitation, but governed by evaporative processes and surface water availability, it may be enhanced by irrigation and changed land use in the Gangetic foreland. Y1 - 2020 U6 - https://doi.org/10.1029/2020GL088420 SN - 0094-8276 SN - 1944-8007 VL - 47 IS - 23 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Brunello, Camilla Francesca A1 - Andermann, Christoff A1 - Helle, Gerhard A1 - Comiti, Francesco A1 - Tonon, Giustino A1 - Tiwari, Achyut A1 - Hovius, Niels ED - Vance, D. T1 - Hydroclimatic seasonality recorded by tree ring delta O-18 signature across a Himalayan altitudinal transect JF - Earth & planetary science letters N2 - Water stable isotope ratios of tropical precipitation predominantly reflect moisture source and precipitation intensity. Trees can incorporate the isotopic signals into annual tree-ring cellulose records, permitting reconstruction of the temporal changes of hydroclimate over decades to millennia. This is especially valuable in the Himalayas where the understanding of monsoon dynamics is limited by the lack of a dense and representative observational network. We have analyzed tree ring delta O-18 records from two distinct physiographic sites along the upper Kali Gandaki valley in the central Nepal Himalayas, representing the wet High-Himalayas and the Trans-Himalayan dryland to the north. Empirical correlations and regression analyses were compared to an in-situ calibrated oxygen isotope fractionation model, exploring the relationships between tree ring delta O-18 and seasonal-mean variability of hydroclimatic forcing at the different locations. For this purpose, gridded precipitation data from the Asian rain gauge dataset APHRODITE, as well as high resolution onsite observations (relative humidity, air temperature, delta O-18 of precipitation and radial tree growth) were used. We found that two distinct sets of meteorological values, reflecting pre-monsoon and monsoon conditions, are needed to reproduce the measured tree ring delta O-18 values from the High-Himalayan site, but that a single set of monsoonal values performs best for the Trans-Himalayan site. We conclude that Trans-Himalayan trees capture long-term changes in strength of the Indian summer monsoon. In contrast, High-Himalayan tree ring delta(18)Orecords a more complex hydro-climatic signal reflecting both pre-monsoon and monsoon seasons with very contrasting isotopic signatures of precipitation. This difference in the two hydroclimatic proxy records offers an opportunity to reconstruct first-order hydroclimate conditions, such as local precipitation rates, and to gain new insights into monsoon timing and seasonal water source determination across the Himalayan orographic region. (C) 2019 Elsevier B.V. All rights reserved. KW - Himalayan hydroclimate KW - seasonal precipitation KW - pre-monsoon KW - monsoon onset KW - oxygen fractionation model KW - dendroclimatology Y1 - 2019 U6 - https://doi.org/10.1016/j.epsl.2019.04.030 SN - 0012-821X SN - 1385-013X VL - 518 SP - 148 EP - 159 PB - Elsevier CY - Amsterdam ER -