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Design of a geographic information supported database for the management of pressurised irrigation systems at the plantation du Haut Penja, Cameroon

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par Chick Herman AZAH
University of Dschang - Agric engineer 2009
  

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4.4 Simulation of the Functioning of the Network

Analysis of the network shows that the total length of the networks is 7 056 m and that the total amount of water that could be transported in the whole network is 5 313 m3/h. With a simulated manometric height of 240 m, the service pressure is 24 bars, while the total head losses in the network is 19.7 m. This gives a loss in pressure of approximately 2 bars. The flow velocity in the various branches vary from 0.8 m/s - 1.1 m/s and are in accordance with the norm ( ) for flow in closed conduits described by Zoungrana, (2002).

The different parameters calculated and the hydraulic characteristics of the network are presented in Appendix III. The flow velocities, the roughness coefficient, the deviation in pressure at the level of the hydrants, the cumulated head losses and the total manometric height are the principal characteristics emphasized on.

The manometric height of the pumping station is obtained as the difference between the retained piezometric elevations upstream and the geometric elevation of the pumping station. Where ever the manometric height considered is less than that which is calculated, the network is said to be mal functional. Where total head losses in the pipe are less than 10% of the service pressure at the pumping station (HMT), and where there exist insufficient pressures in a hydrant, a suppressor will be needed in order to increase the available pressure at the hydrant. Where head losses are greater than 10% of service pressure at the pumping station and that the pressures at the hydrants are sufficient or not, these pressures need to be reduced.

4.5 Spatial Representation of some Queries on the Irrigation System

The database created helps the irrigation manager to inquire information related to irrigation project in GIS environment. Either Permanent data or yearly changing data can be inquired in the database. Some examples of created queries are as follows:

- Parcel irrigated during a given production year

- Crop pattern under selected channel

- Geographical position of plot valves

- Type of irrigation systems in various plots

- Crop pattern under selected parcel

4.5.1 Spatial representation of crop coefficients

The sensibility of crops to water stress is represented spatially as show in Figure 4.4 by considering the crop coefficients for the various production plots and the soil types involved. This helps managers in the localization of plots with critical conditions and hence enhances decision making. Thus, in the case of water shortage and only few plots could be irrigated, the plots with a critical response to stress will be satisfied first. This done through a query of the database and the result presented in the GIS. Figure 4.4 therefore illustrates that in order to take a management decision for irrigation, the plots shaded in green on the map have to be irrigated before those in orange. These plots either have plants with a higher vulnerability to water stress or are found on plots whose soils are of light texture and thus need frequent irrigations.

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