The principles of irrigation practice

Front matter

Edited by L. H. Bailey

THE

PRINCIPLES OF IRRIGATION

Preface

Irrigation and dry-farming are rapidly conquering drought. By these twin arts, bountiful and regular harvests may be gathered in the humid regions during the periodic dry seasons; and in the arid regions, the great "deserts" may be converted into most fruitful fields. Irrigation has a splendid record of success from the beginning of history; dry-farming has only in recent days extended its conquests into the more arid regions; both have become more powerful in conquering drought as modern science has been applied to them.

Successful irrigation-farming is the joint product of the engineer and the farmer. To the engineer is given the heavy and responsible task of constructing properly a permanent system of dams and canals from which water may be drawn; to the farmer belongs the apparently humble but unending and difficult task of using the water in the best manner for crop-production. Both workers are essential for success; but, the work of the farmer determines the permanence and extent of agriculture under irrigation.

Much has been written about irrigation for the engineer, but little for the farmer. The few who have written about farming under irrigation have, most frequently, prepared crop or soil manuals, in which the use of water has formed a minor part. This book is an attempt to develop the principles, so far as present knowledge permits, upon which the correct use of water, by the farmer,

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X PREFACE

must rest. Crop or soil treatments which are not connected directly with the use of water are not discussed or are greatly subordinated. The various aspects of the complex art of irrigation — agricultural, economic, social and legal — will some day receive separate and special treatment; in this volume one line of thought only has been followed — the correct use of water in irrigation.

The aim of this book is to furnish to students and intelligent farmers a modern view of the principles of irrigation practice. Simple language has been used and unnecessary technical terms have been avoided. Obvious matters, and those which vary with local conditions and must therefore be learned by experience, have been eliminated. The beginner in irrigation has been kept in mind; but the book is essentially a manual for those who, whether in arid or humid climates, having cast their lots with irrigation, desire mastery of their work by an intelligent comprehension of the natural laws involved in irrigation-farming. The actual handling of water can be learned only by experience — that is the beginner's heavy lesson; the refinements of irrigation, by which its success at last is measured, come later, and are unknown to many. The man who lives year after year under the ditch, and raises his family there, needs as much if not more help than the pioneer whose chief sorrow is the aggravating self-will of the water as it flows over the newly broken land.

Some subjects have been touched on lightly in this volume because they are more fully developed in the author's book on "Dry-Farming." In fact, that book and this one are a continued study of the water factor in agriculture — perhaps the most important of the physical factors. Schools of agriculture, whether in arid or humid

PREFACE xi

regions, might profitably organize classes in this subject. Classes in fertilizers and related subjects are taught as a matter of course, but the water factor, of greater importance, is given incidental mention in courses on soils or plant physiology. At the Utah Agricultural College it has been found satisfactory to give a half-year course in dry-farming, followed by a half-year course in irrigation practice, the two courses constituting a year's study of the water factor in agriculture.

The irrigation literature of the world has been quite fully examined in the preparation of this book; but, since the work has been done far from large libraries, many important papers have been inaccessible. However, as a possible compensating condition, the work has been done within hearing of the ripple of the irrigation ditch, in the heart of the irrigated section. Free use has been made of all available information, but of especial help have been the magnificent series of irrigation bulletins issued by the Irrigation Investigations of the Office of Experiment Stations of the United States Department of Agriculture. The splendid work of the Bureau of Soils of the United States Department of Agriculture has also been of the greatest assistance. It is a pity that the heated discussion of a theory should overshadow this vast, accurate and remarkable soil work, the like of which, issuing from one institution, is not to be found.

At the end of each chapter has been placed a short list of references for the use of those who desire to carry their studies further. Care has been taken, except in two or three instances, to suggest only such materials as are readily available. These references would make a very good working library on irrigation and may be obtained at a slight cost. In Appendix C is given a brief xii PREFACE

list of books on irrigation. No attempt is made to supplyin this volume a complete bibliography of irrigation.

To friends and colleagues in many parts of the world hearty thanks are offered for valuable help rendered in the preparation of this book. My Utah colleagues, many of whom have been connected with the long experimental study of irrigation at the Utah Station, have given freely of their time and information to make the book accurate and worthy of the cause. I am under particular obligation to Dr. Robert Stewart, Dr. F. S. Harris, Prof. W. W. McLaughlin and Prof. L. A. Merrill, and to Librarian Elizabeth C. Smith, who has kindly gathered irrigation literature from all parts of the world. My brother, Prof. O. J. P. Widtsoe, has in many ways given most valuable help. If this book and its companion volume shall be of service, the first credit belongs to Dr. L. H. Bailey, the Editor of the Rural Series of books, through whose wise and kindly urging these books were written, and the many others, by other hands, which have made available to humanity the great applications of modern science to rural problems.

JOHN A. WIDTSOE.

Logan, Utah.

NOTE. — Unless otherwise stated, wherever 4 'inch" or "foot" of water is used in this book it refers to the depth to which the water would cover the ground.

Table Of Contents

A. INTRODUCTION

CHAPTER I The Meaning of Irrigation 1-7

Annual rainfall, 1 — Seasonal rainfall, 2 — Variations in rainfall, 2 — Conservation of rainfall on farms, 3 — Conditions of dry-farming, 3 — Conditions of irrigation, 4 — Irrigation defined, 4 — Geographical need of irrigation, 5 — Possible extent of irrigation, 5 — Mission of irrigation and dry-farming, 7.

B. THE RELATION OF WATER TO SOILS

Chapter II

Attraction between near bodies, 8 — Soil particles, 9 — The soil-moisture film, 11 — Thickness of film and diameter of particles, 12 — Hygroscopic coefficient, 13 — The wilting coefficient, 14 — Lento-capillary point, 16 — Maximum capillary capacity, 17 — Free water, 17 — Summary, 19.

CHAPTER III The Soil as Water Reservoir 21-39

Irrigated soils are unsaturated, 22 — The movement of soil moisture, 23 — The distribution of soil moisture, 25 — Field moisture capacity, 29 — Water distribution in furrow irrigation, 30 — Effect of hardpan, 32 — Effect of gravel, 34 — Water table near surface, 34 — Soil treatment, 35 — How much water can be stored, 35 — Absorption of water by soils, 38.

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xiv TABLE OF CONTENTS

Chapter IV

The run-off, 40 — The upward movement of water, 42 — Intensity of evaporation, 44 — Conditions determining evaporation, 46 — Mulching to check evaporation, 49 — Self-mulching soils, 52 — Time of cultivation, 53 — Depth of cultivation, 55 — Frequency of cultivation, 58 — Cultivation and soil fertility, 59 — Rolling, 62.

CHAPTER V Soil Changes Due to Irrigation Water .... 64-107

Contraction and moisture film, 64 — Cohesion of soil particles, 65 — Volume changes of soils, 67 — Effect on top soil, 69 — Successive wetting and drying, 70 — Natural packing of soil, 70 — Soil temperature, 71 — Water a universal solvent, 72 — Humid and arid soils contrasted, 73 — Continuous solubility of soils, 74 — Absorption by soils, 76 — Composition of drainage water, 78 — Concentration of soil moisture, 79 — Loss by drainage, 79 — Upward leaching, 81 — Salinity of river waters, 82 — Salinity of lake waters, 86 — Salinity of mineral springs, 86 — Soil moisture and natural waters compared, 87 — Ash constitutents added by irrigation water, 87 — Use of concentrated waters, 89 — Need of water surveys, 90 — Composition of natural waters, 90 — Classification of natural waters, 92 — Plant-food value of irrigation water, 93 — Suspended matter in river water, 95 — Seasonal variation of suspended matter, 98 — Suspended matter added to soil by irrigation, 100 — Suspended matter derived from surface soils, 100 — Composition of river sediments, 101 — Physical effects of sediments, 102 — Cultural treatment of sediments, 103 — Effect of sediments on crop-yields, 104 — Water and soil life, 104.

TABLE OF CONTENTS XV

C. THE RELATION OF WATER TO PLANTS

CHAPTER VI Conditions Determining the Use of Soil Moisture by

Absorption of water by roots, 109 — Transpiration, 110 — The initial percentage of soil moisture, 111 — Distribution of water in the soil, 114 — The effect of time, 115 — The depth of soil, 116 — Physical composition of soils, 117 — Chemical compositions of soils, 118 — Plowing, 120 — Cultivation, 121 — Manuring, 121 — Vigor of plant, 121 — Root-system, 122 — Age of plants, 122 — The kind of crop, 123— The seasons, 124.

Chapter VII

Carbon-assimilation, 128 — Plant age and carbon-assimilation, 129 — Conditions of growth, 130 — The transpiration ratio, 131 — The seasons, 136 — The soil, 137 — Mineral food or soil fertility, 139 — Cultural operations, 141 — The vigor of the plant, 143 — Varying quantities of water, 144 — Maximum yield with given quantity of water, 151 — The nature of the crop, 154 — Summary, 155.

Chapter VIII

Response to irrigation, 159 — Proportion of roots, 160 — Proportion of leaves and stems, 163 — Proportion of heads and grain, 166 — Other plant parts, 169.

xvi TABLE OF CONTENTS

CHAPTER IX The Time of Irrigation 173-188

The ideal principle, 173 — Fall irrigation, 175 — Winter irrigation, 178 — Early spring irrigation, 181 — Irrigation during growth, 182 — Time of irrigating short-season crops, 183 — Time of irrigating long-season crops, 184 — Night versus day irrigation, 187.

CHAPTER X The Method of Irrigation 189-215

Sub-surface irrigation, 189 — Surface irrigation, 193 — Permanent ditches, 196 — Field-ditch or field-lateral method, 198— The border method, 202— The check method, 202— The basin method, 207— The furrow method, 207 — Summary, 214.

Chapter XI

Groups of plant constituents, 217 — Water, 217 — Ash, 219— Protein, 220— Fat, 223— Carbohydrates, 224— Sugars, 224— Starch, 226— Woodiness, 226— Color and flavor, 227— Flour, 227— Cooking value, 228— Effect of cultural treatment, 228.

D. CROPS UNDER IRRIGATION

CHAPTER XII The Use of the Rainfall 231-239

Irrigation supplementary to rainfall, 231 — Crop-producing power of rainfall, 232 — Results of dry-farming, 233 — Crop value of rainfall in irrigation, 233 — Conserving the rainfall, 235- — Distribution of rainfall, 235 — Storing water in the soil, 236 — Cultivation, 237 — Proportion of rainfall conserved, 237 — Relation of irrigation and dry-farming, 237 — Dry-farm homesteads, 238.

TABLE OF CONTENTS xvil

Chapter XIII

Spring vs. fall wheat, 241 — Quantity of wheat to sow, 241 — Method of sowing wheat, 242 — Cultivation of wheat, 243 — Method of irrigating wheat, 243 — Time to irrigate wheat, 246 — Quantity of water for wheat, 248 — Oats, 253— Barley, 255— Rye, 255— Corn, 255— Time to irrigate corn, 258 — Quantity of water for corn, 259 — Rice, 262.

CHAPTER XIV Alfalfa and Other Forage Crops and Pastures 266-285

Alfalfa or lucern, 266 — Cultivation of alfalfa, 268 — Method of irrigating alfalfa, 269 — Time to irrigate alfalfa, 270 — Quantity of water for alfalfa, 274 — Alfalfa seed, 277 — Hay-making crops, 278 — Red clover, 281 — Pastures and meadows, 281.

Chapter XV

Sugar Beets, Potatoes and Miscellaneous Crops . 286-313 Sugar beets, 286 — Method of irrigating sugar beets, 289 — Time to irrigate sugar beets, 290 — Quantity of water for beets, 293— Carrots, 296— Other root crops, 297— Potatoes, 298 — Peas and beans, 301 — Fiber crops, 305 — Hops, 306 — Tomatoes, cantaloupes, etc., 306 — Cabbage, cauliflower, etc., 308 — Asparagus and celery, 309 — Onions and miscellaneous crops, 310.

Chapter XVI

Fruit Trees, Other Trees and Shrubs .... 314-330 Fruit-growing, 314 — Method of orchard irrigation, 315 — Time of orchard irrigation, 319 — Quantity of water for orchards, 322 — Other conditions of orchard irrigation, 323— Nursery stock, 326— Small fruits, 326— Grape-vines, 327 — Plants for ornament and comfort, 328.

xviii TABLE OF CONTENTS

E. MISCELLANEOUS CHAPTER XVII

The Duty, Measurement and Division of Water . 331-370

The duty of water, 331 — Classes of duty, 334 — Determination of duty of water, difficult, 336 — Duty of water in Africa, 338 — Duty of water in Asia, 339 — Duty of water in Europe, 341 — Duty of water in South America, 342 — Duty of water in Australia, 343 — Duty of water in North America, 343 — Bear River Canal experience, 344 — Idaho results, 345 — Miscellaneous results, 345 — The Utah results, 346 — Need of measuring water, 347 — Who shall measure the water? 349 — Classes of measurement, 349— The Cippoletti weir, 353— Divisors, 355— Meaning of the distribution of water, 357 — Methods of Distribution, 358 — Continuous flow, 358 — Continuous rotation, 361 — Distribution on application, 364 — Organization for distribution, 365 — Regulations and records, 368.

Chapter XVIII

Seepage from reservoirs and canals, 371 — Loss from excessive irrigation, 373 — Ground water, 374 — Comparison with humid areas, 375 — Lined ditches — a remedy, 376 — The economical use of water — a remedy, 381 — Drainage — the final remedy, 381 — Alkali defined, 383 — Seepage and alkali, 384 — Upward leaching, 385 — Use of saline water, 387 — Alkali deposits, 388 — Kinds of alkali, 390 — Tolerance for alkali, 392 — Cropping against alkali, 397 — Chemical treatment for alkali, 398 — Scraping the surface, 399 — Tillage against alkali, 399 — Washing out alkali, 400 — Underdrainage — the final remedy, 400.

TABLE OF CONTENTS xix

Chapter XIX

Dry seasons, 407 — Results of irrigation in humid regions, 409 — Methods of applying water, 412 — The duty of water, 413 — Sources of water, 413 — Water-conservation methods, 414 — Value of sewage water, 414 — The use of sewage, 415 — Factory and mill waste, 417.

Chapter XX

Clearing and breaking the land, 419 — Laying-out the farm, 420 — Leveling the land, 423 — Making farm ditches, 426 — Gates and checks, 434 — Ridging and furrowing, 439 — Mulching the soil, 440 — Measuring the flow of water, 441.

Chapter XXI

The antiquity of irrigation, 445 — The Christian era to 1800, 449 — Irrigation in recent times, 451 — The founding of modern irrigation in America, 454 — The growth of American irrigation, 457 — The Union Colony of Colorado, 460 — The United States Reclamation Service, 461 — The United States Department of Agriculture, 464 — The experiment stations, 466 — The Irrigation Congress, 470.

Chapter XXII

Permanent Agriculture under Irrigation . . . 472-476 The big irrigation problem, 472 — The spirit of irrigation, 473 — No essential difference between irrigation- and humid-farming, 473 — History assures permanence of irrigation, 474 — The question of plant-food, 474 — Some advantages of irrigation, 476 — Finally, 476.

xx TABLE OF CONTENTS

Appendix

A. Water Constants 477

B. Discharge over Cippoletti's Weir 478-483

C. List of American Books on Irrigation . . . 484

LIST OF ILLUSTRATIONS

Fig. Page

Brigham Young Frontispiece

1. Progressive averages of annual rainfall 3

2. The limited water supply makes it unlikely that more than one-

tenth of the land will be irrigated 5

3. The value of water in an arid land 6

4. Soil is a mixture of particles of very varying size 10

5. The moisture film surrounding a soil particle 18

6. Flooding new land 24

7. Distribution of water in soil immediately after an irrigation .... 27

8. Distribution of water in soil under furrow irrigation 31

9. Penetration of roots of prune tree 36

10. Evaporation usually exceeds rainfall 44

11. Relation between temperature and evaporation 48

12. Evaporation losses from soils protected with mulches of differ-

ent depths 51

13. Orchard well cultivated to prevent evaporation 57

14. Mulching the garden with a hand cultivator 61

15. Adhesion of hairs due to water 65

16. Cracked river sediments showing volume changes due to water 68

17. Midsummer snow in the tops of the mountains 75

18. Badly corroded ditch due to excessive fall 95

19. Walled ditch to prevent erosion of easily "washed" soil 97

20. Daily discharge of Malheur River 99

21. Daily discarge of Mackenzie River 99

22. Deposit in field of suspended matter from irrigation water 102

24. Determining the transpiration ratio 135

25. Yield of dry matter of cereals with varying quantities of water . . 149

26. Yield of dry matter of cereals per inch of irrigation water 149

27. Crop-producing power of 30 acre-inches (wheat) 152

28. Crop-producing power of 30 acre-inches (alfalfa) 152

29. Crop-producing power of 30 acre-inches (sugar beets) 153

30. Effect of little, medium and much water on wheat 160

31. Proportion of grain and straw with varying irrigations 168

32. Plan of a sub-irrigated farm 191

33. Lee's sub-irrigation system 192

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xxii LIST OF ILLUSTRATIONS

Fig. Page

34. A permanent ditch in an orange grove 196

35. Plan of field-ditch irrigation 197

36. Flooding from ditches running down the steepest slope 198

37. Flooding from field ditch 198

38. Flooding with aid of canvas dam 199

39. Laterals made in field and dammed with small piles of manure

for next year's irrigation 200

40. Plan for border irrigation 201

41. Border method of irrigation 202

42. Irrigating cherries under check system 203

43. Rectangular check method of irrigation 204

44. Contour check method of irrigation 205

45. Filling checks with detachable pipes .• 206

46. Orchard irrigation by basin method 207

47. Orchard irrigation by basin method 207

48. Grading of interior of basins to prevent water from coming in

contact with trees 208

49. Furrow irrigation 208

50. Furrow irrigation of young alfalfa 209

51. One-way furrow irrigation 210

52. Furrowing land 211

53. Standpipe supplying furrows with water 212

54. Zigzag furrows to insure uniform distribution over soil 213

55. Another type of zigzag furrows 214

56. Lath-box for distributing water to furrows from head ditch. . . . 214

57. Yield of crops due to rainfall 235

58. Irrigating wheat 244

59. Canvas dam to check water 245

60. Irrigated wheat 249

61. Irrigated oats 251

62. Plan of rice irrigation 254

63. Yield vs. water (wheat) 256

64. Producing power of 30 acre-inches 257

65. Yield vs. water (corn) 261

66. Irrigated corn in Arizona 262

67. Plan of irrigating an alfalfa field 270

68. Temporary county fair building constructed of baled alfalfa hay 271

69. An alfalfa field in Nevada 272

70. Yield vs. water (alfalfa) 276

71. Flooding pasture land 280

72. Irrigating young alfalfa 281

73. Irrigated cane in Kansas 283

74. A sugar beet field 287

LIST OF ILLUSTRATIONS xxiii

Fig. Page

75. Loading sugar beets in factory bins 288

76. Irrigating potatoes 293

77. Yield vs. water (sugar beets) 295

78. Plan of potato irrigation 298

79. Irrigating potatoes 299

80. Irrigated field peas 302

81. Irrigated celery 304

82. Irrigated pumpkins 305

83. Irrigated onions 307

84. Yield vs. water (potatoes) 309

85. Irrigated Egyptian cotton 311

86. Irrigating cantaloupes 312

87. Irrigating an apple orchard 318

88. On the upper canal 322

89. An irrigated prune orchard 325

90. An irrigated date palm orchard 328

91. Canal crossing river in an inverted syphon 332

92. Looking down the Bear River Canal 333

93. Lateral outtake from large canal 335

94. Headgate of a canal 340

95. A cable measuring station with automatic gauge 348

96. Lyman rectangular weir 350

97. Longitudinal section of Lyman's weir 351

98. Cippoletti weir 353

99. Details of Cippoletti weir 354

100. Scale to be screwed on side of Cippoletti weir 355

101. Divisor attached to Cippoletti weir 356

102. Turnout and measuring weir 357

103. Device for diverting a constant quantity of water 357

104. The need of storing water in reservoirs 359

105. Rise of ground water from irrigation 374

106. Chain puddler. Used in making canals watertight 376

107. Modified chain puddler 377

108. Wooden stave pipe carrying irrigation water 378

109. Lateral lined with concrete 379

1 10. Cement-lined main canal 380

111. Pumping plant 382

112. Drainage of irrigated lands by intercepting drains 383

113. Structure of an alkali spot 386

114. Quaternary Lakes of the Great Basin. Sources of alkali deposits. 389

115. Effect of a strong solution of potassium nitrate on protoplasm. 392

116. Vegetation on alkali lands 393

117. Alkali rising in spots on irrigated pasture 396 xxiv LIST OF ILLUSTRATIONS

Fig. Page

118. The annual rainfall of Milan compared with that of humid and

arid districts in the United States 408

119. Comparative yields of strawberries, irrigated and unirrigated 409

120. An irrigation plant in Pennsylvania 412

121. Distribution of water on Craigentinny Meadows, Edinburgh. . 416

122. Section of cement flume 420

123. Section of V-shaped flume 420

124. Wooden flume , ... 420

125. Section of rectangular flume 420

126. Flume with lateral gate 421

127. Buck scraper 421

128. Leveler or float 422

129. Shuart grader 422

130. Soil auger 423

131. Lateral plow 423

132. V-crowder 424

133. Building a ditch 424

134. Typical forms of farm ditches 425

135. Concrete drop in ditch 426

136. Drop in flume 427

137. Distributor for hose 428

138. Attaching hose to distributor 428

139. Leveling device 428

140. Lateral headgate 429

141. Concrete gate 429

143. Board dam 429

144. Canvas dam 430

145. Canvas dam with opening 430

146. Metal dam 430

147. Distribution of water from flume to furrows 430

148. Distribution through wooden tubes 430

149. Lath check 431

150. Conducting water down inclines in concrete pipes 431

151. Roller furrower 432

152. Utah lay-off and pulverizer 434

153. Robinson's adjustable corrugator and renovator 434

154. Ridger in check and basin irrigation 435

155. Ridger in check and basin irrigation 435

156. Furrower in action 435

157. Cultivator 436

158. Cultivator attachments 436

159. Beet cultivator attachments 437

LIST OF ILLUSTRATIONS xxv

Fig. Pago

160. Alfalfa renovator 438

161. Clod crusher, pulverizer, leveler and smoother 438

162. Frieze water register 439

163. Device for measuring miner's inches 440

164. Cross-section of canal for measurement of flow 440

165. Current meters 44 1

166. Grant-Mitchell meter 442

167. Leveler in action 443

168. Sagebrush land 446

170. Shadof of Egypt or Paecottah of India 448

171. Caravan crossing the plains in early irrigation days 455

172. Irrigation canals are cut through the mountains 459

173. Major J. W. Powell 463

174. Completed diversion dam of the Truckee Carson, Nev., pro-

ject of the United States Reclamation Service 465

175. Steam power digs the modern canals 466

176. View of the irrigation plant of the Utah Experiment Station. . . 467

177. Dam of Salmon River project, Idaho, built by private enter-

prise 468

178. Plant for the study of the measurement and division of water. 469

179. Work for a man 475

ACKNOWLEDGMENT FOR ILLUSTRATIONS

The illustrations in this book are either original or taken from the publications of the United States Department of Agriculture, the United States Geological Survey, and the state experiment stations, with the following exceptions, which have been secured from

Bark, Don H., Irrigation Investigations, United States Department

of Agriculture. Figs. 50, 58, 59, 71, 72, 76, 91, 108, 168, 177. Blinn, P. K., Colorado Experiment Station. Figs. 38, 86, 156. Blanchard, C. J., United States Reclamation Service. Figs. 6, 49,

69, 87, 93, 110, 174.

Bonebright, J. E., Montana Experiment Station. Figs. 18, 39, 60,

61, 167.

Cutler, Thomas R. Fig. 75 Forbes, R. H. Figs. 22, 66, 83, 90, 117. Gillette, C. P., Colorado Experiment Station. Fig. 178. Greeley Commercial Club. Fig. 79.

Harris, F. S., Utah Experiment Station. Figs. 16, 19, 26. Jardine, W. M., Kansas Agricultural College. Fig. 73. Jarvis, O. W., Superintendent, Southern Nevada Experiment

Farm. Fig. 85.

Johnson Company, The, Salt Lake City. Figs. 14, 17, 82, 92, 94,

171, 172, 179.

McLaughlin, W. W., Irrigation Investigations, United States

Department of Agriculture. Figs. 52, 88, 151. Ogden Commercial Club. Fig. 81.

Redland's Commercial Club. Fig. 34.

San Jose Commercial Club. Fig. 13.

Santa Clara Commercial Club. Fig. 89.

Smith, George Otis, United States Geological Survey. Fig. 173. Winsor, Luther M., Utah Experiment Station. Figs. 68, 80, 175.

Grateful acknowledgment is made of the assistance rendered by the above persons.

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THE PRINCIPLES OF IRRIGATION PRACTICE

Chapter I The Meaning Of Irrigation

Water, soil, air and sunshine are the four great groups of physical forces that determine the growth of plants. For the production of plant crops, all of these must be present and active. Water, therefore, is essential to plant-growth and crop-production.

The water that falls upon the earth in the form of rain and snow is the source of all the water used in agriculture. To be of value in plant-growth, this water must be stored, for longer or shorter periods, in the soil in which plants are growing. Whenever the soil becomes too dry or too wet, or if the total quantity is insufficient or too large, plant-growth is hindered. It is the concern of agriculture to maintain in the soil the proper quantity of water during the growth of crops.

1. Annual rainfall. — Water, in the form of rain or snow, falls over the whole earth. No place is known where some rain does not fall at some time during the year. However, the quantity that falls varies greatly in different regions. Over the so-called deserts the annual rainfall is often less than 5 inches, while in various places near the seashore or among the mountains, the annual rainfall exceeds 100 inches and occasionally reaches 600 inches or more, as at Assam, India. From district to district, the world over, the quantity of water that falls annually upon the farmers' fields is different.

2. Seasonal rainfall. — Moreover, the time of the year at which the rain comes is not everywhere the same. In some localities the rain falls chiefly in summer, during the season of crop-growth; in others, during the spring; in others, during the fall, and in yet others, during the winter. Going eastward from the Pacific seaboard, this difference in seasonal rainfall is well brought out. In California, the heaviest rainfall comes during midwinter; in the Great Basin, in early spring; on the eastern slope of the Rocky Mountains, in late spring, and on the Great Plains, near midsummer. The annual rainfall may be fairly large in a given locality, but it does not necessarily fall at the time that plants are growing.

3. Variations in rainfall. — Added to these variations in the quantity of total rainfall, and in the distribution throughout the year, is still another: The same quantity of rain does not fall in the same place from year to year. In one year there may be a large precipitation, and in another a very light one; and the time of the year of the heaviest downfall may be shifted somewhat from year to year.

True, the variations are not large. The driest year seldom receives less than two-thirds the rainfall of the wettest year, and usually it receives more. True, also, so far as our records show, the average rainfall over a certain locality for ten or twenty years is practically constant. The average rainfall at any one place is nearly invariable, although distinct variations occur in successive years. (Fig. 1.)

MEANING OF IRRIGATION 3

4. Conservation of rainfall on farms. — It is the business of the farmer so to handle his farm that the largest possible proportion of the rain that falls may be made to enter the soil, and to remain there until needed by plants, unless, indeed, the rainfall is too heavy, when provision must be made for relieving the soil of the harmful surplus. To accomplish this, the farmer must resort to methods for

Mew &7gr/o/7d Oh/'o \/a//ey

1 83 4-/8 #5 tc o l\

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A r^

J\

A

A^

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Fig 1. Progressive averages of annual rainfall (1834-1906).

preventing, or reducing largely, the loss due to the run-off, evaporation and seepage. Where the annual rainfall is fairly large and well distributed, these methods are not applied extensively; but where the annual rainfall is light or is not in the growing season, moisture-conserving methods are indispensable. There are relatively few localities on earth where special efforts to conserve the rains for plant-growth are not rewarded by large crop yields. The smaller the annual rainfall, the greater is, naturally, the return for careful moisture-conservation. 5. Conditions of dry-farming. — If the annual rainfall be very light, it frequently happens that, with the best available tillage methods, the water conserved in the soil is so small that only certain crops can be grown, and of these crops maximum yields are not obtained. The annual rainfall, below which special moisture-conserving methods are necessary, is different at different places. Ordinarily, where the rainfall is 20 inches or less, the special waterconserving methods of dry-farming must be used. Where the average temperature is high; where heavy winds blow largely; where the soils are shallow or infertile, or where other water-dissipating factors are especially active, the methods of dry-farming must be employed, even if the rainfall reaches 25 or 30 inches annually. On the other hand, with our present experience, we are obliged to admit that when the annual rainfall is less than 10 inches, our present methods of water-conservation are not sufficient, except in a few special localities, to make profitable crop-production possible.