634.2.6 Bench Terraces
Terraces of different types have been extensively and often successfully used in
watershed management for soil and water conservation on hilly land, up to 40 %
slope. Bench terraces are perhaps the oldest type of terrace. They are used primarily
in areas where the supply of agricultural land is limited and where population pres-
sure has forced cultivation up steep slopes. Early bench terraces were constructed
by carrying soil from the uphill side of a strip to the lower side so that a level step
or bench was formed. The steep slopes below the terraces were stabilized by vegeta-
tion or neatly-fitted stonework (Fig. 8 ). The construction of terraces has continued
in recent years, particularly in countries with limited land and high population
pressure.
Terraces reduce both the amount and velocity of water moving across the soil
surface, which greatly reduces soil erosion. Terracing thus permits more intensive
cropping than would otherwise be possible. A horizontal surface in terrace reduces
runoff and maximizes water infiltration into the soil. If the soil surface is kept tilled
and free of vegetation except for the desirable crop, almost all of the rain falling on
the terrace will be used for crop growth. In regions with low rainfall, soil water can
be stored for long periods provided the soil surface is kept tilled or mulched and
vegetation free. Thus, it may be possible to store up to two years of rainfall to obtain
one cereal crop. If there are several terraces down a hillslope, it may be possible to
grow a crop on alternate terraces each year (Prinz 2002 ). Infiltration terraces can
support soil water storage to make a crop viable each year where total rainfall is
sufficient. It can also provide for perennial tree crops which develop extensive root
systems, as typically occurs for olive trees in semiarid Mediterranean areas. In some
cases, the cropped terraces are downslope of a runoff area and may infiltrate more
water than that provided directly by rainfall, as occurs in the Meskats which are
traditional in Tunisia (Missaoui 1996 ).
Fig. 8 Bench terraces:
Rice paddies in China.
Source: WILDCHINA
( 2012 )
Water Harvesting in Dry Environments