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Crop coefficients and irrigation losses

Crop coefficients and irrigation losses

The crop coefficients account for the fact that potential evapotranspiration is dependent of the crop type and the stage of growth. This is achieved by multiplying the input time series of potential evapotranspiration with the crop coefficient. For dry uncropped soils, the crop coefficient will tend to zero while well cropped fields reaching full growth potential could have crop coefficients greater than 1. Further information about how to estimate the crop coefficients is given in 6. Monthly values of crop coefficients can be specified.

The monthly magnitude and distribution of losses to evaporation, seepage, and overland flow should correspond to the design values of the irrigation scheme unless observations have shown a different distribution.

Saturated hydraulic conductivity values for soil textural classes

Figure: Values of saturated hydraulic conductivity Ks for eleven textural classes from six sources: (1) Li et al. (1976)4; (2) Brakensiek (1979)1; (3) Brakensiek et al. (1981)2; (4) McCuen et al. (1981)5; (5) Cosby et al. (1984)3; (6) Rijtema (1969)7. The ratio between the largest and the smallest estimate has been calculated for columns 1-5 and 1-6, respectively.

  1. Brakensiek, D.L.: Comments on 'Empirical Equations for some soil Hydraulic Properties' by Roger B. Clapp and George M. Hornberger, Water Resources Research, 15 (4), 989-990, 1979. ↩

  2. Brakensiek, D.L., Engleman, R.L. and Rawls, W.J.: Variation within texture classes of soil water parameters, Trans. ASAE, 24, 335-339, 1981. ↩

  3. Cosby, B.J., Hornberger, G.M., Clapp, R.B. and Ginn, T.R.: A statistical exploration of the relationships of soil moisture characteristics to the physical properties of soils, Water Resources Research, 20 (6), 682-690, 1984. ↩

  4. Li, R.-M., Stevens, M.A. and Simons, D.B.: Solutions to Green-Ampt infiltration equations, J. Irrig. and Drain. Div., Amer. Soc. Civil. Eng., 102 (IR2), 239-248, 1976. ↩

  5. McCuen, R.H., Rawls, W.J. and Brakensiek, D.L.: Statistical analysis of the Brooks-Corey and Green-Ampt parameters across soil textures, Water Resources Research, 17, 1005-1013, 1981. ↩

  6. Rawls, W.J., Brakensiek, D.L. and Saxton, K.E.: Estimation of soil water properties, Transactions of the ASAE, 25, 1316-1320, 1982. ↩

  7. Rijtema, P.E.: Soil moisture forecasting, Nota 513, Instituut voor Cultuurtechniek en Waterhuishouding, Wageningen, The Netherlands, 1969. ↩