Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 327

Chapter 9
Drainage of Agricultural Lands
Contents
9.1 Concepts and Benefits of Drainage ...................... 329
9.1.1 Concepts .............................. 329
9.1.2 Goal and Purpose of Drainage .................... 329
9.1.3 Effects of Poor Drainage on Soils and Plants .............. 329
9.1.4 Benefits from Drainage ....................... 330
9.1.5 Types of Drainage .......................... 330
9.1.6 Merits and Demerits of Deep Open and Buried Pipe Drains ....... 332
9.1.7 Difference Between Irrigation Channel and Drainage Channel ...... 334
9.2 Physics of Land Drainage .......................... 334
9.2.1 Soil Pore Space and Soil-Water Retention Behavior ........... 334
9.2.2 Some Relevant Terminologies .................... 335
9.2.3 Water Balance in a Drained Soil ................... 338
9.2.4 Sample Workout Problem ...................... 340
9.3 Theory of Water Movement Through Soil and Toward Drain .......... 341
9.3.1 Velocity of Flow in Porous Media ................... 341
9.3.2 Some Related Terminologies ..................... 341
9.3.3 Resultant or Equivalent Hydraulic Conductivity of Layered Soil ..... 342
9.3.4 Laplace's Equation for Groundwater Flow ............... 345
9.3.5 Functional Form of Water-Table Position During Flow into Drain .... 346
9.3.6 Theory of Groundwater Flow Toward Drain .............. 346
9.3.7 Sample Workout Problems ...................... 347
9.4 Design of Surface Drainage System ..................... 349
9.4.1 Estimation of Design Surface Runoff ................. 349
9.4.2 Design Considerations and Layout of Surface Drainage System ..... 349
9.4.3 Hydraulic Design of Surface Drain .................. 349
9.4.4 Sample Work Out Problem ...................... 350
9.5 Equations/Models for Subsurface Drainage Design ............... 351
9.5.1 Steady-State Formula for Parallel Drain Spacing ............ 351
9.5.2 Formula for Irregular Drain System .................. 355
9.5.3 Determination of Drain Pipe Size ................... 356
9.6 Design of Subsurface Drainage System .................... 356