Gravimetric sampling density for accurate crop water use calculation under cracking Gezira clay soil in Sudan
2007
Abdelhadi, A.W., Agricultural Research Corporation, Land and Water Research Center, Wad Medani (Sudan) | Rahmatalla, E., Agricultural Research Corporation, Land and Water Research Center, Wad Medani (Sudan)
Although gravimetric soil moisture measurement method is destructive, time consuming and laborious, it is considered as one of the most accurate methods. The method is widely used in the central cracking clay plain known as Gezira clay (covering about 26 Million ha) to monitor soil moisture pertinent to studies of crop water requirements (CWR). Actual CWR calculation depends on simple water balance equation due to lack of deep drainage, capillary rise and runoff losses from experimental plots. Thus soil moisture depletion between irrigation cycles represents actual crop water use. This is equal to CWR provided that soil moisture was not limiting during the irrigation cycles. However, it is reported that small number of gravimetric samples within the experimental plots resulted in erroneous calculation of CWR pertaining to large moisture variations. An experiment was conducted for two seasons (2004 -2005) at Gezira Research Farm using a Sorghum crop, variety Wad Ahmed. The purpose was to provide more information on the spatial variability with respect to different irrigation regimes and at various depths as well as to determine minimum number of samples required at two levels of accuracy (± 1.0 and 2.5%). The experimental design was randomized complete block design RCBD with three water intervals (weekly, fortnightly and every three weeks). Intensive gravimetric samples (9 to 12) were taken from an area of about 50 to 96 m2 two to three days after each irrigation and on weekly basis until next irrigation cycle starts. The results showed no specific trend on soil moisture variation with depth, irrigation numbers or timing. The sampling density requirements increased tremendously as the level of accuracy was raised to ± 1% compared to ± 2.5%. It was concluded that the calculation of crop water use has to be based on large irrigation plots with intensive and frequent gravimetric soil moisture samples
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