Medium and large-scale water harvesting projects where the water is generated have a variety of disadvantages such as:

The above advantages and disadvantages will be discussed in the following paragraphs on examples from Jordan.

Surface and groundwater quality starts to develop in the atmospheric air where precipitation water forms and reacts with gases of the atmosphere, mainly composed of nitrogen, oxygen, carbon dioxide, and a variety of low-concentration gases differing from one area to another. In addition, atmospheric natural and man-made dust and sooth contribute to the composition of precipitation water ( Salameh et al., 1991 ; König, 1994 ). Table 1 gives the composition of rainwater in the different areas in Jordan; Figure 7 illustrates the geographic coordinates of these stations.

The mountainous stations such as Amman, Salt and Irbed receive precipitation with low EC values whereas, in the Plateau area further east the dust-loaded atmosphere contributes to the salt content of precipitation (Ruseifa, Azraq, QAIA, Khalidiya and Muwaqqar) ( Figure 6 , Figure 7 ). Towards the Jordan Rift Valley in the west, the salt content increases also as a result rainfall interception and re-evaporation in the atmosphere due to the thick atmospheric air column and generally higher temperature in the Rift Valley than on the highland areas by an average of around 10˚C ( Khashman, 2002 ; Salameh et al., 1991 ; König, 1994 ).

Upon contact with the ground surface, rainwater starts to react with the rock and soil covers and dissolve minerals, mainly because rainwater naturally contains the acidic bicarbonates (HCO₃), which have capabilities to dissolve salts. Along the water overland flow, (channels, creaks, wadis, rivers) and during infiltration into soils and rocks, the water reacts with the encountered rocks and soils, especially, because it dissolves more carbon dioxides from the air and from the soils. Table 2 and Table 3 list the composition of floodwater along wadis pouring into the Jordan Rift Valley and wadis flowing in the Plateau area. The Plateau floods are generally composed of floodwater alone and hence their low salinity, compared to floods flowing towards the Jordan Rift Valley, which mix with base flow waters having higher salinities than flood precipitation water as explained above. Nonetheless, the salinity expressed in EC terms remains low, generally, below 250 µS/cm.

It can be stated here that the salinity of rainwater increases gradually from the highlands with altitudes of 700 - 1000 masl (Amman, Irbed, Salt, Karak, Shoubak) eastwards, towards the desert area as a result of, generally, more dusty atmosphere and lower ground surface altitudes of 500 - 750 masl (Ruseifa, Azraq, QAIA,

Table 2. Flood flow composition along the plateau wadis (EC in µS/cm, NO₃ and PO₄ in mg/l and all others in meq/l) High NO₃ and to a certain extent PO₄ concentrations can be correlated with the EC values indicating the effects of urbanization and accompanying waste production in the catchment areas of these watercourses.

Table 3. Flood flow composition along wadis pouring into the Jordan Rift Valley (EC in µs/cm, NO₃ and PO₄ in mg/l and all others in meq/l).

Khalidiya and Muwaqqar), ( Figure 8 and Figure 9 ). In addition, the salinity increases also towards the Jordan Valley area (Deir Alla) because of its low latitude at 235 m below sea level and the re-evaporation (precipitation interception) of parts of the rainwater during precipitation leading herewith to concentration of salts in the falling water than in the escaping water (out raining water). The pH of the water is 7 - 8, quite normal for earth alkaline-carbonate water.

Water collected in dams in Jordan, even where the catchment area is not or, only very sparsely urbanized contains higher amounts of dissolved materials than

floodwater. That is because dams receive base flow waters with increased salt contents and because water collected in dams is exposed to both evaporation and reaction with the accumulated bottom sediments, which is in addition to urban, irrigation and industrial leachates ( Table 4 ).

As can be seen from Table 4 , the water collected in the dams of Yarmouk, Wadi Arab, Kufranja, King Talal, Kafrain and Shueib have increased salinities. These dams receive urban wastes and treated and untreated wastewater ( Figure 10 ). Floods reaching Tannur dam contain mining byproducts of phosphates and Zarqa Main, Shueib, Kafrain and King Talal dams receive phosphates of household use and therefore, they are suffering of eutrophication processes ( Figure 11 ).

Harvested water in areas, where it generates, is of far lower salt content than that collected in dams (Compare Tables 1-4 ). Water harvested in high altitude areas, is saved from mixing with higher salinity base flow and other impaired water qualities along the flow course and from water reactions with dams’ bottom sediments ( Salameh, 1996 ).

Low salinity water of precipitation and floods is of higher value and productivity value when used in irrigation, and can in addition, be used in sectors requiring its high qualities such as drinking uses.

Water at high altitudes possesses potential energy compared to low altitudes. Hence, capturing water at high altitudes is quasi-gaining energy, which in case of natural floodwater becomes lost by friction with the flow channel and erosion and transportation of the sediment load. Capturing one cubic meter of water at 100 m above a certain level is equivalent to gaining 1 Mega Joule or around 100 Newton and that saves soils and rocks from erosion and transportation. In addition, if the harvested water is used in irrigation at these higher altitudes, it means energy gains in the form of potential energy, saving the transportation cost of agrarian products.

Jordan is located in the semi-arid to arid climatic zone, where day to night temperatures differ by about 10˚C, in addition to hot summers and cool winters, where temperatures rise in summer days beyond 40˚C and drop to below zero in wintertime. Hence, physical weathering is very strong and results in the disintegration of rocks. Thus, making the small rock pieces available for transportation by rain and floodwater. In addition, wind storms coming from the desert areas of the Arabian Peninsula in the south and east and Sinai Peninsula in the southwest carrying dust affect the area and these storms deposit their solid particles there ( Salameh et al., 1991 ; König, 1994 ). These particles are easily eroded and transported with the floodwater in a downstream direction. Capturing floodwater in high altitude areas results in the deposition of their sediment loads in the capturing structure and by that reducing downstream siltation of floodwater collection facility.

Here, it is worth mentioning that one of the major problems facing existing and planned dams in Jordan is siltation ( Salameh et al., 2024 ). For example, King Talal Dam has been filled by silt to around 25% of its storage capacity after 46 years of its construction, and Wadi Al-Arab dam to 18% after 40 years ( Table 5 , JVA open files). Muwaqqar three successive weirs distanced less than 200 m from each other along Wadi Mugheir with capacities of 15.000 - 25.000 m³ silted up totally after around 10 years of their construction.

Harvesting water at high altitudes reduces the quantity of floodwater flowing to downstream areas and causing damage to human life, natural environment, and to infrastructure. In Jordan, some big floods have resulted, during the last decade, in human and animal casualties, damage to buildings, dam spillways, streets, bridges and to the natural landscape and habitat ( Figure 12 ). Water harvesting in high altitudes will certainly reduce flooding risks in downstream areas.

Figure 6 shows that the potential evaporation rates in the Highlands and Plateau areas of Jordan decrease with increasing altitude. Therefore, irrigation water requirements at high latitudes, where rain and floodwater can be harvested, are lower than those requirements in the downstream areas ( Taimeh, 2016 ; Evans, 1990 ; NRA & GTZ, 1977 ). This means that a unit of irrigation water at high latitudes produces more agricultural products than at lower latitudes due to higher evaporation rates. For example, the potential evaporation rate in Azraq area is around 1050 mm/yr whereas, in the headwater area further north, near the borders to Syria, it is only 900 mm/yr.

In addition, some salinity-sensitive crops such as asparagus and hydroponic plants require low water salinity; <450 µS/cm, which is provided by flood water in the upper reaches of water catchment areas.

Water harvesting and use in the high altitude areas, where the water generates creates needed new jobs, reduces unemployment and poverty, and fosters peoples’ identification with their land and hence, with the country, which contributes to security, safety and sustainability. Knowing that indigenous population owns and inhabits these areas, compared to the areas irrigated by dams’ water, where workers are generally foreigners.

The hydropower energy of some dams in Jordan produces electricity, such as the case of King Talal Dam. Although the produced quantity of energy is small, harvesting some of the head floodwater will reduce the hydropower production of such dams. Nevertheless, water harvested at higher altitudes possesses potential energy, which can be exploited in an efficient way. In addition, water storage at higher altitudes, for whatever use purpose, is an energy-gain process. Worth mentioning here is that, each cubic meter of water has a potential energy of 1 Mega Joule or around 1/3 kw∙hr for every 100 m difference in head.

Wadis and river courses in the Plateau area of Jordan are generally influent, which means water flowing along them infiltrates into the ground to recharge the underlying groundwater bodies. The longer the watercourse, the more permeable its bottom rocks, the larger the wetted wadi course area and the lower the wadi gradient, the more floodwater infiltrates and recharge the groundwater. Therefore, capturing floodwater in upstream areas may deprive the groundwater in the downstream areas of some recharge. Hence, the importance of detailed studies in this context, knowing that capturing of floodwater in storage facilities in high altitude areas also results in groundwater recharge. This, forces developers of water harvesting projects, to carry out detailed studies to weight advantages versus disadvantages of upstream water harvesting on the natural groundwater recharge aiming not to cause damages to the environment and downstream water uses.

The higher the discharged rate of flood flow the larger the wetted area along the watercourse and in its destination collection facility, whether the latter is a natural or artificial water body. Therefore, upstream water harvesting lowers the wetted areas along watercourses and in terminal lakes and hence, it lowers the soil moisture, the biodiversity, and the natural plant production, which serves grazing and production of desert vegetation. The natural service provided by flood flows require intensive studies during the planning stages of water harvesting projects, especially because the natural vegetation may include area-specific species, medicinal and aromatic plants, which may be very special species and restricted in their distribution to certain watercourses. Therefore, remediation measures must be developed, planned, and implemented aiming at conserving natural vegetation and biodiversity.

Rain and head-flood water, as discussed above, are of very low salinity and generally of very good quality. When flowing down along their natural courses, they serve as diluting factors for other downstream waters containing higher salt contents or pollutants, such as animal manure, treated wastewater, polluted spring water, and lower quality water stored in dams or weirs. Harvesting and consuming the harvested water in upstream areas reduces the downstream water quality. Therefore, detailed water quality appraisals must be carried out before decisions are taken to harvest flood and rainwater in the up-stream areas. Corrective measures can be developed and implemented to protect the environment.

Examples: floodwater reaching King Talal dam has an EC value of 390 µS/cm, the treated wastewater in Khirbet es Samra ~2000 µS/cm, the floodwater quantity ~20 MCM/yr, and the treated wastewater quantity ~120 MCM/yr. If all the floodwater becomes captured in the upstream area, the salinity of the water stored in King Talal Dam will increase by, around 230 µS/cm, which is expected to reduce the agricultural water productivity of King Talal Dam water in the Jordan Valley area.

In many areas in Jordan, especially those along wadi courses draining into the Rift Valley area, floodwater has been, since decades, stored in dams for the different use purposes. However, along many other wadis and small catchments floodwater is still being lost to desert playas where it salinizes and evaporates, to areas outside the borders of the country, or to the open sea (Red Sea) and the Dead Sea.

Here, it can be recommended to capture all not used floodwater, after studying the effects of capturing that water on the other environmental elements, such as soil moisture, grazing land, natural groundwater recharge, biodiversity, etc.

Harvesting floodwater, which is presently under use in the downstream areas, must underlie rigorous, hydrological, hydrochemical, economic, social and environmental studies on its viability, and its benefits, which should be weighed against disadvantages. Fair compensation of existing downstream users and environmental elements must be of a major concern in the design phase of any water-harvesting project otherwise; the benefits of such projects remain illusionary.