Land use in the Republic of Yemen is particularly dependent on the climate, in particular on the natural water supply associated with the climate. Various irrigation methods are used in land use.
Due to the country’s location south of the Tropic of Capricorn, it has a subtropical to tropical climate with two rainy seasons per year. However, they do not bring reliable amounts of rain. Agriculture is therefore often dependent on additional artificial irrigation.
Due to the special geographical conditions, Yemen can be divided into three climatic regions. Because of the rainfall, the growing areas of agriculture are primarily in the highlands with their large depressions and in the upstream mountain ranges, in the parts of the Tihamah close to the mountains and also in the deeply cut wadis.
Climate and land use
Due to the country’s location south of the Tropic of Capricorn, it has a subtropical to tropical climate with two rainy seasons per year. However, they do not bring reliable amounts of rain. The agriculture is therefore often dependent on additional irrigation.
Due to the special geographical conditions, Yemen can be divided into three climatic regions. Because of the rainfall, the cultivation areas for agriculture are primarily in the highlands with their large depressions and in the upstream mountain ranges, in the parts of the Tihamah close to the mountains and also in the deeply cut wadis, in which surface water and available groundwater are the prerequisites.
Coastline and Tihamah
Summers are extremely humid in the low-precipitation coastal strips on the Red Sea and in the southern and eastern provinces (the Tihamah). Here there is a hot and humid tropical climate with summer temperatures above 40 °C and in winter between 30 °C and 35 °C with an extreme humidity between 65% and 90%. At night it hardly cools below 20 °C (Fig. 3). The coastal area is characterized by dry savannah, in which salty plants, in the wadis and on the alluvial fans in front of it, acacias, tamarisks and dump palms grow.
Tropical and subtropical fruits such as citrus, mango, papaya and banana thrive in the deep valley of the mountain waste. Daily balancing currents in the air cause clouds and fog to form over the Tihamah during the day at heights of up to 5000 m, which flow away to the east through gaps in the upland ascent.
On the mountain slopes, due to the increasing precipitation (including fog) and decreasing temperatures, a distinction is made between three levels of vegetation: the myrrh belt (200 to 1 300 m) with stem succulents, such as the bottle tree, the evergreen fog belt (1300-2100 m), where among others the coffee grows, and the euphorbia thornbush belt in the trench depressions (long valleys) between the upstream mountain ranges and the highlands.
The largely uninhabited settlement boundary between the residents of the Tihamah and those of the highlands lies in the trench depressions between the highlands and the upstream mountain ranges. The former do not settle above 1,000 m, the latter not below 1,300 m above sea level. An artistic slope terracing begins above 1300 m.
Mountain Yemen highlands
The highlands of Mountain Yemen are characterized by strong sunlight with relatively moderate daytime temperatures in winter (20 °C-25 °C) and great heat in summer (25 °C-30 °C). The relative humidity varies between 20% and 50%, the maximum temperatures reach up to 38 °C in June / July. However, the nights cool down noticeably, in December / January the temperatures can even drop below freezing point at night (Fig. 9).
Lee side of the mountains
In the rain shadow of the leeward side of the mountains , towards the Great Arabian Desert (Rub al-Chali), the vegetation is decreasing and is finally missing completely in the sandy and salt deserts. Temperatures there can rise to over 45 °C in summer, and between 30 °C and 35 °C in winter.
During the months of March / April and July to September in the highlands and on the ascent of the mountains there are often short but strong thunderstorm rains (often heavy rain, mostly in the afternoons) due to monsoons or trade winds, which in the high mountain regions up to 1000 mm and in the interior Highlands in the rain shadow still produce 300 mm of precipitation per year, which flows off in tidal waves through the wadis. This temporally and spatially different natural water supply led to different methods of irrigation between Tihamah and Rub al-Kali:
The farmers collect additional water on sterile rock surfaces, which they feed into their fields to compensate for any precipitation deficits. One advantage of this irrigation is the regular fertilization of the soil with the minerals from the rocks carried along by the incoming water.
Flood or Sayl irrigation
Incoming flood waves are directed from the beds of the wadis onto the field plots via deflection dams. The ancient dam of Saba (near Marib), which was destroyed several times by tidal waves (most recently finally in 542 AD), functioned according to this principle.
A constant supply of water from springs is used. However, this is rarely possible.
In the absence of precipitation and the lack of spring water, water from cisterns or from the groundwater is used. Groundwater is conveyed through draw wells and increasingly through pumps. This is especially true in the desert areas in the north and east, where the desert climate is always dry and hot.
The only usable areas without additional irrigation that can be operated in rain -fed farming are on the southwestern edge mountains and on sandy soils in the Tihamah. There they cultivated as crops traditionally mainly millet and date palms, but increasingly large area cotton, papaya, mango and citrus with additional irrigation.
The precipitation, which often occurs in the mountain regions as thundery heavy rain, quickly leads to soil erosion on the steep slopes . That is why large parts of the cultivated land areas in the mountain regions are artfully terraced up to an altitude of over 2500 m. Grain, coffee and increasingly qat, a common intoxicant, are grown on these cultivated terraces.
The coffee, which was still pending the transfer of the coffee tree in the 18th century in the European colonies as a significant export product of Yemen is now grown only in small quantities in the fog zone and exported.
In place of coffee and other crops in the light and almost everywhere in the highlands easily to cultivate Catholic amine-containing stepped Qat -Strauch (Catha edulis Forsk) from the plant family of celastraceae (Celastraceen). The daily consumption of the young shoots of the Yemeni everyday drug Qat by around 80% of the population (including women) means a serious economic and social problem for the economy and for consumers. In many parts of the country, qat has become the main crop at the expense of traditional cultures. According to unofficial estimates, qat cultivation alone accounts for 30% of Yemen’s economic volume.
Qat is now also seen by the Yemeni government as one of the main obstacles to the country’s development and the reduction of its enjoyment as a central concern of Yemeni politics.
Springs and deep wells enable extensive irrigation oases in the inland basins of the highlands , in which all kinds of vegetables, alfalfa, fruit, potatoes, tobacco and wine thrive.
In the valley cuts of the foothills above the frost line (1800-3700 m), mainly tree fruits such as pears, peaches, apricots, plums, figs and walnuts thrive. On the hillside terraces, cereals such as Dhurra millet, wheat and barley predominate.
Where wadis leave the eastern mountain slopes in the foothills of the Rub al-Chali, we find large agricultural and irrigation oases . From there, the water of the high tides moves eastward in groundwater currents from the highlands. It collects in Wadi Najran and Wadi Hadramaut, where there are other irrigation oases with fields and date palm cultures along the irrigation ditches.
Frankincense trees , myrrh and coconut trees are cultivated in the southwest and on the southern coast.