The importance of observed gradients of air temperature and precipitation for modeling runoff from a glacierized watershed in the Nepalese Himalayas

Abstract

The performance of glaciohydrological models which simulate catchment response to climate variability depends to a large degree on the data used to force the models. The forcing data become increasingly important in high-elevation, glacierized catchments where the interplay between extreme topography, climate, and the cryosphere is complex. It is challenging to generate a reliable forcing data set that captures this spatial heterogeneity. In this paper, we analyze the results of a 1 year field campaign focusing on air temperature and precipitation observations in the Langtang valley in the Nepalese Himalayas. We use the observed time series to characterize both temperature lapse rates (LRs) and precipitation gradients (PGs). We study their spatial and temporal variability, and we attempt to identify possible controlling factors. We show that very clear LRs exist in the valley and that there are strong seasonal differences related to the water vapor content in the atmosphere. Results also show that the LRs are generally shallower than the commonly used environmental lapse rates. The analysis of the precipitation observations reveals that there is great variability in precipitation over short horizontal distances. A uniform valley wide PG cannot be established, and several scale-dependent mechanisms may explain our observations. We complete our analysis by showing the impact of the observed LRs and PGs on the outputs of the TOPKAPI-ETH glaciohydrological model. We conclude that LRs and PGs have a very large impact on the water balance composition and that short-term monitoring campaigns have the potential to improve model quality considerably.

Impact of debris cover on glacier ablation and atmosphere–glacier feedbacks in the Karakoram

“…In addition, exposed debris alters near-surface meteorological fields and their elevational gradients, which are often key modeling parameters used to extrapolate forcing data from a point location (e.g., an automatic weather station) over the rest of the glacier surface (e.g., Marshall et al, 2007;Gardner et al, 2009;Reid et al, 2012). The lapse rate in air temperature at lower elevations is more than one degree steeper in DEB, as a result of surface temperatures exceeding the melting point and a higher net turbulent transfer of sensible heat to the atmosphere that produces higher near-surface air temperatures, and is higher than values reported for smaller debris-covered glaciers (Reid et al, 2012) and in the eastern Himalaya, where the monsoon circulation is more dominant (Immerzeel et al, 2014b). eling approach on the Baltoro glacier, and with the measured rates reported by Mayer et al (2010).…”

Modeling Slope Environmental Lapse Rate (SELR) of temperature in the monsoon glacio-hydrological regime of the Himalaya

“…Higher inter-annual variability of nival-glacier regime SELR as 20 observed for Beas and Sutlej basins indicate more complex processes driving these changes and raise questions regarding its use for modeling snow and glacier melt, especially for modeling future runoff and glacier fluctuations. Various researchers have shown that the glacier and snowmelt estimation by degree day method is highly sensitive to the near surface lapse rate Marshall et al, 2007 andImmerzeel et al, 2014). This result points towards the need for revisiting the benchmark glacier monitoring strategy (Fountain et al, 1997) for mountain glaciers.…”

Modified temperature index model for estimating the melt water discharge from debris-covered Lirung Glacier, Nepal

“…Collected data are distributed on each grid by using temperature, and precipitation gradients. In this study the precipitation gradient is used as described by (Immerzeel et al, 2014) and the temperature gradient is calculated from station data available from Yala and Kyangjing temperature data.…”