Land use changes

Objectives: Determine the effects of land use and climatic changes on the sustainability of high-altitude oases.

Contact PersonProf. Dr. Andreas Bürkertand Prof. Dr. Eva Schlecht, Institute of Crop Science, University of Kassel, Germany

Email:tropcrops[at]uni-kassel[dot]de

Scientific staff: Dr. Eike Luedeling, Dr. Stefan Siebert

Study area: Northern Oman

Duration: 2005-2007    

The crop composition of high-altitude oases of Al Jabal al Akhdar is markedly different from that of lowland oases. Instead of date palms, the terraces belonging to these settlements feature tree crops more typically found in temperate and cool subtropical regions, such as pomegranates, peaches, apricots and walnuts. All of these tree species depend on the occurrence of cool temperatures in the winter to break their dormancy and initiate fruiting. These so-called chilling requirements are typically quantified in chilling hours by summing up the number of hours during the winter, when temperatures range between 0 and 7°C. The unique compositions of the orchards of Al Jabal al Akhdar, and the inability of lowland farmers to grow the same tree crops indicates that these high-altitude systems lie in an environmentally marginal location for the cultivation of these crops. Consequently, they might be vulnerable to climatic change.

For assessing the effects of climate change on the production of temperature and cold-loving subtropical fruits and nuts, hourly temperatures were measured at the oases of Al 'Ayn, Qasha' and Masayrat ar Ruwajah between 2005 and 2008 (Figure 1). Based on daily temperatures recorded at the nearby weather station at Saiq and modeled daylength, equations were developed by Partial Least Squares regression to estimate hourly temperatures based on historical records of daily temperatures. This method was then used to compute hourly temperatures since 1983 and calculate the number of chilling hours for each winter season. In two climate change scenarios, increases of the mean annual temperature by 1 and 2°C were assumed, and temperatures modeled using a stochastic weather generator. This process yielded an estimate of the likely distribution of chilling hours to be expected in the future.

The number of chilling hours appeared to be on the decline in Oman's high altitude oases over the period of this study (1983-2008; Figure 2). Chilling requirements of most tree crops were not met in most years at the lower oases, while at Al 'Ayn, the highest oasis, conditions were still sufficiently cold for most trees. Both future scenarios, however, indicated that chilling conditions are likely to become marginal to insufficient even at this location (Figure 3). It should be noted that the exact chilling requirements of the varieties cultivated in Oman are unknown, and that under the prevailing temperature conditions in the oases, chilling hours might not be a good indicator of winter chill (Luedeling et al., in preparation).

To accommodate inevitable interannual fluctuations in their water supply, oasis farmers in Oman typically cultivate annual crops on some of their terraces, in addition to the perennials that provide the majority of food and fiber. In years of limited water availability, these fields can be left uncultivated, allowing farmers to conserve water without affecting their valuable date palm or pomegranate orchards. This traditional practice indicates that the proportion of the oasis that is cropped with annual crops is an indicator of the stability of an oasis' water supply, and that changes to this proportion might put oasis sustainability at risk.
High-altitude cropping systems in Al Jabal al Akhdar (Figure 1) have been affected by multiple changes in recent decades, such as the introduction of new species and varieties, the abandonment of cereal cultivation and the emigration of oasis farmers leading to the decay of many terraces. The net balance of all of these changes is likely to have influenced overall water consumption of the oases, and the objective of this study was to quantify these changes.
Based on recent and historic aerial and regular photographs (1978 vs. 2005), land use of the oases of Al 'Ayn, Al 'Aqr, Ash Sharayjah, Qasha' and Masayrat ar Ruwajah was mapped, and water consumption of each oasis was modeled based on crop evapotranspiration. Water requirements were then assigned to either annual or perennial crops to provide an estimate of the amount of water that is not essential for maintaining oasis orchards. All such estimates were compared to the amount of water available to oasis farmers by quantifying spring flow at all oases.

Land use changed substantially in some oases, especially at Qasha', where most of the former cereal plots were converted to orchards. Similar changes occurred in parts of Ash Sharayjah, while at Masayrat ar Ruwajah, most of the former lime orchard was converted to annual crops (Figure 4). The net balance of these changes was favorable for Masayrat ar Ruwajah and Al 'Ayn, where more water was dedicated to annual crops in 2005 than in 1978. For the other oases, the proportion of orchard vs. field crops increased during this period, indicating that oasis resilience to water scarcity had decreased substantially. Especially at Qasha' and Al 'Aqr, spring flow now appeared insufficient to reliably supply irrigation water to all fields (Figure 5). Additional pressure on regional water resources is likely to originate from increased domestic water consumption in the oasis and the new town of Sayh Qatanah, as well as from newly developed tourism infrastructure.