5.1 Precipitation
As we saw in the previous chapter, one of the earliest cornerstones of soil physics was laid in 1856 by the French engineer, Henry Darcy, whose work focused on water filters and groundwater aquifers. Shortly thereafter a German, Ewald Wollny, became perhaps the first scientist whose published work focused squarely on soil physics. Wollny’s experiments as early as 1874, were the first to quantitatively describe how raindrops falling on bare soil degraded the soil structure and how the vegetative canopy provided a vital protection against raindrop impact [1]. His subsequent experiments revealed that the vegetative canopies of common crops could intercept from 12-55% of the total rainfall. These discoveries, and his subsequent studies on runoff and erosion, make Wollny one of the pioneers of soil physics and hydrology and an early leader on the important issue of soil and water conservation. Our study of soil water balance process will begin where Wollny, and arguably soil physics itself, began – by considering precipitation and the interactions between raindrops, vegetative canopies, and the soil.
5.1.1 Precipitation Amount
The global pattern of precipitation amounts over land is arguably one of the most important influences on the Earth’s coupled human and natural systems. Average annual precipitation ranges from <100 mm in the great deserts of the world to >2000 mm in the equatorial rainforests (Fig. 5‑2). In fact, average annual rainfall exceeds 5000 mm (16 feet!) in a few locations, such as eastern India, where mountains lift warm, moist ocean winds cooling the air and resulting in precipitation. This process of orographic lift produces relatively high precipitation amounts on the upwind side of the mountains and relatively low precipitation areas, known as rain shadows, on the downwind side. The Earth’s precipitation patterns are clearly reflected, not only in the global distribution of natural ecosystems, but in the patterns of human population, as well (Fig. 5‑3). The majority of the world’s cities with >1,000,000 people are located in regions with average annual rainfall between 500 and 2000 mm. If climate change produces substantial changes in precipitation patterns, then shifts in human population patterns or increased water transfers by humans or both are likely to result. In fact, large scale water transfer projects have already begun, such as the enormous “South-to-North Water Diversion” project in China, which allows transfer of 25 x 109 (109 = billion) cubic meters of water per year from the Yangtze River basin in south China to Beijing and other locations in north China across a distance of more than 1,000 km [2].
5.1.2 Precipitation Intensity
The afternoon of June 22, 1947, was warm and humid in Holt, Missouri, a small farming community northeast of Kansas City, and a rainstorm, which had dropped 10-60 mm of rain across eastern Nebraska and Kansas, was approaching from the northwest [3]. The residents of Holt had