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3.4-5: Orientation - Planning - Action
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3.4-5: Orientation - Planning - Action
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41: climate 2 |
This chapter presents the collected informations on structural, functional and normative systems that are relevant for forestry in Yemen. It starts with the Physical Environmental Factors (4.1), especially Climate and Rainfall, the dominating factor what concerns their influence on the local culture. Under The Natural Environmental Factors (4.2) the Degradation of the Vegetation is shown and the remaining Natural Forests of Yemen (4.3) described.
Then the Socio-Economic background (4.5), decisive in what concerns functional aspects, is analysed, starting with the Rural Economy (4.4). Not only Wood Trade is here of interest, but especially the Indigenous Knowledge on the Management of the Natural Environment. Most important for management, that is for steering, are the social structures and especially ownership (The Waqf Begeli). On the normative level we have to deal with The Tribal Law ('urf) and Ownership regulations. As those Norms are grounded in History and Religion, those basic cultural ideologies are analysed as well (6) to get a better understanding of the situation. The ways of dealing with norms are described in the Policy Analysis (4.7). A condensation of the Recommendations is done in the last chapter (5).
As the objective of all activities undertaken in Yemen was "DEVELOPMENT", the approach was handled in a reflexive, self-critical, iterative way. The aim was not to develop methodologically perfect assessment techniques (that no Yemeni is interested in), the aim is action. If action does not happen, the development efforts have been in vain! So the problem is - what knowledge is needed to motivate action, action perceived as needed. Knowledge (as technology-) transfer with such an objective has to be a two-way process. It has to be an exchange of knowledge done through dialogue. "Information" has to be seen from different perspectives: A) what information is useful for the purpose, B) how and where can this information be obtained and C) how is information used, how does the partner deal with knowledge (methodologically and motivation-wise).
This (C) seems to me the main part of "development research" - how does the knowledge "arrive", how is it taken up by the local partners, how are the conclusions translated into action. For that reason, problems of data collection, processing, interpretation and use make part of this chapter, as well as a few dropped and failed approaches. Those are as important, if not more important than successes, as both depend on varying sets of factors and only knowledge and analysis of both will create a clear and true picture of the development potentials.
4.1 The Physical Environmental Factors
4.1.1 The Climate of Yemen
4.1.1.1 Temperature, Humidity and Winds
Yemen is situated at the fringe of the tropics. The yearly average of 18.3 C° , the absolutely frost free, chilling limit is the lower limit to classify a country as tropical. This limit is exceeded in all areas below 2000m (-1800), it is massively exceeded in the Tihama with about 30 C° . The heat limit for most agricultural plants (e.g. qat) is exceeded below the altitude of 1000m. ][ s. Kopp p. 38:]
The main influence on temperature is exerted by the altitude. Hudeidah is at sea level, Taiz about on 1400m, Sana'a on 2200m. The areas above 3000m form the tropical alpine zones. The typical species is the Juniper (Juniperus excelsa, arab. 'Ar-'ar). All the altitudes above 2000 m might suffer some frost, but only for a few hours during winter nights (until 4am at Sana'a). The minimal temperature in daytime is even for Sana'a in December substantially above zero.
The two graphs above show clearly, that Hudeidah is very hot and damp. The curve for Sana'a demonstrates the influence of the rainy season. In march-april the temperature does not increase, but humidity does so significantly.
The potential evapotranspiration (pET) [Straub (1986) p. 43-44:] differs by area [mm/year]:
Riyan 1626
Khormaksar/Aden 1817
Perim 1882
Kamaran 1937
- and by formula used:
At Tur 1696 [Turc]
1869 [Radiation Formula]
2016 [Bilaney-Criddle]
2509 [McCulloch 1965/Penman]
What gets definitely clear from those values, is the fact, that Yemen is dry. Dryness is relative. Its importance for plants is best measured as comparison between the potential evapotranspiration and the available rainfall. Yemen is arid in the Eastern Desert and the Tihama, semiarid in the Highlands and subhumid at certain areas of the Serrat, as e.g. Bura', Ibb and Taiz.
Winds [s. Glaser, in: Rahtjens; Flohn (1965)]: The major factors influencing winds is the atmospheric circulation. In winter the dominant wind is the NE monsoon, in summer the SW-monsoon. The winds in the Tihama (in the mountains it depends on the local topography), blow from october to april from SSE, in summer from NNW. Especially during summer, when loads of dust are in the air, it is rather uncomfortable and many Tihamis suffer from infected eyes. Dust clouds are blown from the Tihama up to the Highlands, where they even form "clouds" in summer. In the Serrat the dominant factor is the passat, winds that blow according to temperature differences from the mountains to the plains, from the sea to the land, during daytime, the opposite direction during the night. Those winds do have a practical influence on the wellbeing of people (including field experts). Generally the wind is changing direction in the evening between seven and nine p.m. - as long as there are no clouds. If there is no wind in Tihama during the night, the mosquitos make live a hell to the camper.
There are two rainy seasons for most of the Yemen. Namely march/april and july/august. Only at Houf and on Socotra rain is depending on the summer monsoon alone. Rain normally falls in the form of thunder-showers, heavy and short. - But even that is not relyable. In 1989 e.g. it was raining at Sana'a for two weeks almost without interruption. The mud houses have been soaked and even cement ceilings started to leak through. For the following two years there was no rain, neither in spring nor in summer!
Climatologically and botanically the most special area of Yemen is surely Jebel Houf in the extreme south-east, at the border to Oman. During the summer monsoon (July-August) it is covered with dense clouds. The hills of Houf, reaching about 1400m, trap the humidity. Fine drizzle is falling day and night. During the rainy season of July and August this area makes an incredible impression. The sky is dark grey. It is not really raining but there is a constant fine drizzle all day and night. Everything is soaked with water. The coastal hills on the road from Al Gheidha to Houf are green with mosses and ferns, the sea storming. Waves over two meter high are rolling onto the beach as if they wanted to climb up to the Houf mountains. Patches of fog moving hither and thither - Hamlet would be at home here! This, for Arabia rather queer climate, is due to the monsoon. The heavy storms cause the lower and colder levels of sea water to move upwards. This so called "upwelling" phenomenon causes the average temperature of the sea water during those summer months to dropp for some 10 C° . A fact that can easily be felt. While in the coastal Tihama of the Northern Yemen the climate gets the nastier (more hot and more damp) the closer you get to the sea - on the Mahra coast the climate gets the cooler the closer one moves to the sea!
Due to the heavy "wet deposition", estimated by Bilaidi (Forest Research Station al Kod) at 1000mm (!), the area is almost inaccessible during this time. The residents and their cattle move to the lower fringes of the forest. The best time to visit the area is September, as drizzle stopped but all plants are flowering.
Socotra gets the same monsoon-storms during summer, as strong - but rather dry. But there the main rains fall in winter, some 130mm/y at the airport. No measurements of the surely substantial wet deposition is available.
Generally rainfall depends on orography. It is highest at Ibb-Yarim with an estimated 800-1000mm (see comments on meteorological data). Second along the escarpment: Taiz, Jebel Reyma, Jebel Bura', Manakha, Mahwiit; all of them with about 600 (-800) mm per year. Rainfall is decreasing from Manakha to the North and from Taiz to the south, as well from the Serrat to the coast west and south (100mm) and towards the Eastern Desert. Sana'a receives about 170mm, Jebel Lawz (50 km east of Sana'a) 130mm. At Ma'rib it must be already substantially below 100mm a year, not to speak about the Eastern Desert (Ramlat as Sabatain and Rub al Khali). All areas with less than 200 mm do not support forests. Trees can only survive there along the wadis or in ecological niches, where runoff water accumulates. This can be seen most impressively in the Hadhramout.
The graphs on the following three pages show the trends:
Yemen has high rainfall zones at Ibb and Hajja. There are two clines, depending a) on topography and b) on the west-east (or south-north) extension. Rainfall is increasing on the Serrat with increasing altitude. On the leward side of the mountains it is decreasing the more one moves inland.
Two details: The bow of the isohyet to the west, south of Sana'a, is due to the Wadi Siham, in the wind-shadow of Bura', Raima and Harraz (see rainfall at Jebel Bura', further down).
On Khulaidis map (Graph 5) the 200mm isohyet looks to be much too much eastward.
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