The functional relationships for DUand ea [Eqs. (5.135) and (5.136)] indicate that, for each set of field characteristics, there exists an optimal value for the slope 50. However, a given average slope may correspond to a precisely leveled field or one with uneven microtopography. Consequences of irrigating an uneven basin are shown in Fig. 5.16. Differences in advance time are small because of the high inflow rate, but differences in recession time are large, giving widely varying infiltration opportunity time. This results in uneven infiltrated depths, greatly affecting irrigation performance, particularly for basins and furrows.
Recent land leveling advances have been made in both computational procedures [41-43] and the use of laser control of land grading equipment.
The leveling precision can be estimated from the standard deviation of field elevation differences to the target elevation:
where Sd is the standard deviation of field elevation (m), hi is the field elevation at point i (m), and htj is the target elevation at the same point (m). When precision laser leveling is used, it is possible to achieve Sd < 0.012 m whereas conventional equipment does not provide better than Sd = 0.025 m.
Poor land leveling particularly affects the distribution uniformity in furrowed level basins supplied through an earthen ditch at the upstream end. Differences in furrow i=1
entrance elevations cause differences in inflow rates and volumes entering each furrow. Even though the water may be redistributed after the advance is completed through a ditch at the downstream end, DUis highly influenced by Sd. Combined effects of individual inflow rates (qin) and basin lengths (L) are shown in Fig. 5.17, relating the Sd of furrow entrance elevations with the ratio between the actual DU and the maximum value for DUexpected when land leveling would be optimal; DUmax.
Maintenance of precise laser leveling requires tillage equipment that conserves the landform after land grading and the adoption of appropriate tools for furrow opening. An economic analysis of land leveling impacts given by Sousa et al.  shows how poor land grading leads to lower yields and higher maintenance costs.
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