Landwirtschaft und klimatische Trends im zentralen Yarlong Tsangpo-Tal, Tibet

Abstract

Agriculture and climatic trends in the central Yarlong Tsangpo valley, Tibet The Yarlong Tsangpo valley in southern Tibet is the centre of Tibetan farming. About 75% of the area is cultivated with local barley cultivars with some additional spring and winter wheat. With the exception of the Xigaze basin, cultivation is limited by seasonal flooding to higher ground on terraces, debris fans and small upslope areas in the valley. Generally favourable thermal and solar conditions in the subtropical high-altitude valley at elevations between 3,500-3,900 m are offset by low precipitation and high evapotranspiration rates. Previous temperature-based estimates of evapotranspiration rates underestimate the actual conditions considerably; annual Penman-Monteith evapotranspiration totals are estimated at 1,194 und 1,364 mm for Lhasa and Xigaze. Based on long-term water balance calculations (1954-1990), the seasonal water demand of barley and wheat is estimated at 460-730 mm (580-770 mm) and 530-840 mm (660-920 mm) at Lhasa (and Xigaze, resp.). Mean total plant-available water during the growing season amounted to only 54-59% and 44-49% of the seasonal water demand (wheat and barley, resp.) at Lhasa and Xigaze, resp. and decreased by 12% and 4-8% during that period. Estimated water balance data therefore point to increasingly arid conditions in the region. A shift towards cultivating more drought-tolerant barley cultivars instead of wheat, and raising the effectiveness of irrigation systems are possible counter measures. Zusammenfassung: Das Yarlong Tsangpo-Tal in Südtibet ist das Zentrum des tibetischen Ackerbaus. Lokale Gerstensorten nehmen neben Sommerweizen und Winterweizen ca. 75% der Getreideanbauflächen ein. Mit Ausnahme des Beckens von Xigaze beschränken sommerliche Hochwasser den Anbau überwiegend auf die Nutzung höhergelegener Terrassen, Schwemmfächer und vereinzelter Hanglagen. Den für ein subtropisches Hochtal mit mittleren Höhenlagen von 3.500-3.900 m günstigen thermischen und solaren Bedingungen stehen geringe Niederschläge und hohe Verdunstungsraten gegenüber, die von bisherigen temperaturgestützten Berechnungen der Verdunstung erheblich unterschätzt wurden. Jährliche Penman-Monteith-Evapotranspirationssummen für Lhasa und Xigaze betragen 1.194 und 1.364 mm. Auf der Basis von langjährigen Wasserbilanzberechnungen ergibt sich zwischen 1954 und 1990 eine hohe Schwankungsbreite des saisonalen Wasserbedarfs für Gerste (460-730 mm bzw. 580-770 mm) und Weizen (530-840 mm bzw. 660-920 mm, jeweils in Lhasa und Xigaze). Damit werden in Lhasa im Mittel nur 54-59%, in Xigaze nur 44-49% des saisonalen Wasserbedarfs durch Niederschlag und Bodenwasser gedeckt. Diese Wasserbilanz hat in Lhasa im Beobachtungszeitraum um je 12% und in Xigaze zwischen 4% und 8% (Gerste bzw. Weizen) abgenommen. Die berechneten Wasserbilanzen weisen damit auf eine Verschiebung der Klimaverhältnisse zu arideren Bedingungen hin. Bevorzugter Anbau der trockentoleranteren Gerste anstelle von Weizensorten und eine Instandsetzung der Bewässerungssystem...

A glacier inventory for the western Nyainqentanglha Range and the Nam Co Basin, Tibet, and glacier changes 1976–2009

“…analysed data of the station Amdo, and recorded multi-year oscillations in precipitation from 1965 to the mid1990s, followed by a trend to increasing precipitation since 1995. The July precipitation even slightly decreased until the mid 1990s (Thomas and Chen, 2002). Annual precipitation in the last decade was 50.6 mm (about 12%) above the average annual mean during the period 1965-1994.…”

“…The same tendency was observed by Shi et al (2006), Kang et al (2007b), and Chen et al (2009) for the stations adjacent to our study region. Pan evaporation showed stable or slightly increasing values until the mid 1990s (Thomas and Chen, 2002) and a decrease thereafter (Liu et al, 2010).…”

Climatic change on the Tibetan Plateau: Potential Evapotranspiration Trends from 1961–2000

“…The TP was identified as the only large contiguous region in China were potential yields were declining. This is underlined by a regional study where decreasing precipitation and increasing PET rates in summer have been shown to affect the largest crop growing region in central Tibet (Thomas and Chen, 2002) by lowering potential yields by up to 12% during the last 50 years. Despite generally decreasing PET rates water balance studies therefore point to generally decreased water availability for field crops during the last decades.…”

“…From 1951 to 1993 annual PET rates in the eastern part of the TP showed both increasing and decreasing trends of more than 20 mm/decade (Thomas, 2000a). In the Yarlong Tsangpo valley (Central Tibet) July PET rates have decreased moderately by 7.5 mm/decade between 1954-1993(Thomas and Chen, 2002. For the upper reaches of the Yellow River Li et al (2000) reported an annual increase of 3.3 mm during the summer months (May to October) from 1981 to 1998.…”

“…For a long time publications of estimates of potential evapotranspiration (PET) rates in Tibet were restricted to generalized maps (Kayane, 1971;Henning and Henning, 1984) or to individual stations (Henning and Henning, 1981). PET estimates based on temperature data alone (Thornthwaite, 1948;Lieth et al, 1996;Walter, 1955) have been shown to be inaccurate underestimating actual evaporative conditions on the TP considerably (Thomas and Chen, 2002). Only recently a number of estimates based on the Penman-Monteith equation have become available (Thomas, 1999(Thomas, , 2002) that were also included in the FAOCLIM data base (FAO, 2001).…”