Land and Watershed Management - Practices of Irrigation and On-farm Water Management - page 199

Watersheds are delineated by United States Geological Survey (USGS) using a
nationwide system based on surface hydrologic features. This system divides the
country into 21 regions, 222 subregions, 352 accounting units, and 2,262 catalogu-
ing units. A hierarchical hydrologic unit code (HUC) consisting of 2 digits for each
level in the hydrologic unit system is used to identify any hydrologic area. The
6-digit accounting units and the 8-digit cataloguing units are generally referred to
as basin and subbasin. There are many states that have defined down to 16-digit
HUCs.
6.4 Land Grading in Watershed
6.4.1 Concept, Purpose, and Applicability
6.4.1.1 Concept
Land is the important basic resource that supports the production of all agricul-
tural commodities, including livestock, which is dependent on land to produce the
grain and forage consumed. Land grading is the manipulation of the land sur-
face from its natural condition. It is reshaping or restructuring the existing land
surface for planned grades as determined by engineering survey, evaluation, and
layout. “Planned grade” is depended upon the planned activities such as irrigation
(in target method) and/or subsequent drainage, erosion control, specific crop culti-
vation (e.g., horticultural crop or cereals such as rice). Land improvement consists
of betterments, site preparation and site improvements that ready the land for its
intended use.
Altering the surface of agricultural crop land is a common practice used primar-
ily to improve the drainage of water from a field or to increase the efficiency of
surface irrigation. There are several common names for this practice, including land
smoothing, land leveling, land grading, and land forming. Although each of these
practices generally involves altering the land surface, there are distinct differences
in the methods used to level the surface of the land (ASAE, 1998 ) .
Generally, there are two basic types of practices that alter the surface of agri-
cultural land: land smoothing and land grading. Land smoothing involves shaping
the land to remove irregular, uneven, mounded, broken, and jagged surfaces with-
out using surveying information. This operation would typically be performed by
a tractor pulling a land leveler or other type of smoothing implement. Land grad-
ing is the operation of shaping the surface of land to predetermined grades so that
each row or surface slopes to a drain, or is configured for efficient irrigation water
application.
Land grading also involves smoothing the land surface to eliminate minor depres-
sions and irregularities, but without changing the general topography. Irregular
micro-topography can cause severe crop loss.
6.4.1.2 Purpose
The purpose of grading is to provide more suitable topography for facilitating
crop culture, irrigation, surface runoff control, and to minimize soil erosion and