Since most of Outremer is semi arid if not arid, water is the prime necessity. Rainfall in Outremer fals mainly in the winter. In some of the higher elevations - Moab and eastern Aqaba, and the mountainous areas of Homs, Tripoli, Antioch, and Armenia - much of the precipitation comes in the form of heavy snowfalls. Then, for six to nine months of the year, it seldom rains. Streams are often intermittent at best, and in the deserts the are often totally dry - except when it rains. Without vegetation, there is nothing to hold back the water, which accumulates into dry rivers called "wadis" in tremendous flash floods. This then spews out into lower flat areas where is soon evaporates. The trick in these areas is to manage the water, so that it is available even in times when the skies are clear for months on end.
Even in the less dry areas, the semi-arid and comparitively lush places like the Judean hills, or the hills of Moab, people use the most elementary water storace device, the bottle cistern. This is a vertical hole carved into the rock, with water channels draining to it. The cistern has a narrow "bottleneck" opening, and is lined inside with plaster. Other cisterns are shaped like boxes, rectangular, with a narrow top to help prevent evaporation. These can be huge or small, or anything in between.
Dams across wadis can do one of two things - divert floods into rock-cut cisterns, or retain water in the valley. The first is more common, and less difficult to build, but both types are known from ancient times. The Nabateans used wadi dams to supply much of Petra's water a thousand years before the Crusaders came.
Seeps and springs were also used - both come from an aquiferous layer underground. In seeps, water continually beads up and trickles down an exposed rock face. In springs, the water gushes naturally from the ground. Rock-cut channels were used to collect the water from seeps and divert it into collecting tanks. Springs had walls built around them to form collecting pools. Both served to settle the dirt out of the water, and had at least one, and possibly more, exits into channels.
Two types of channels were used besides cutting troughs into the bedrock. Surface aqueducts were constructed of U-shaped segments of stone or clay, mortared together into channels. Care had to be taken with the slope, so that the water ran down them at a natural pace. Qanats were a series of vertical shafts connected by gently sloping underground tunnels, which lead from an underground aquifer far into the desert. The water in the tunnels can be accessed at any of the shafts, and the underground construction minimizes evaporation.
With judicious use of the water collected, irrigation made the deserts bloom. The Nabateans, for example, grew grapes and olives in the Negev and the Arabah with directed irrigation from cisterns and dams. The Franks re-discovered this technology in Petra, where they built a castle in our timeline, and which they resettled in the Outremer timeline. In return, irrigation enabled better water retention through vegetation, which enabled more vegetation, which increased water availability. Water enables life, and life creates water.