OCR Text |
Show PART II PRESENT STATUS OF IRRIGATION Many factors influence the amount of water consumed by crops. These vary with locality and time. The more important influences are climate, water management, type of crop, and plant growth characteristics. The magnitude and intensity of rainfall affect the amount of irriga- tion water required during any season. Under certain conditions the rain- fall may "be lost through evaporation or surface runoff before it can be utilized by the crop. For other conditions a large percentage of the moisture enters the soil and is available for crop consumptive use. Consumptive use of water by crops in any particular locality is in- fluenced more by the amount of solar radiation than any other factor. Long-term temperature records are good indicators of solar radiation and the corresponding water use by plants. The growing season, also closely tied to temperature, is an important factor and is frequently considered to be the period between killing frosts. For many annual crops and loca- tions, however, the growing season is much shorter as these crops are usually planted after the frosts are past and mature before they recur. At higher elevations where the growing season is short, many of the an- nual crops, mostly small grains, do not mature and are cut for hay. For most perennial crops, namely, grass hay and pasture, growth starts as soon as the maximum temperature stays well above the freezing point and continues throughout the season despite later freezes. Many of the hardy crops, especially grasses, mature even though growing season temperatures repeatedly drop below freezing. The stage of a crop's growth influences its consumptive use rate. From the time of crop emergence and continuing to maturation, the rate of water use increases as the vegetative cover develops. Where adequate moisture is available in the region, the highest use occurs during the time of maximum vegetative cover. After the crop matures, the consump- tive use rate decreases. The term "crop irrigation requirement" as used in this appendix is defined as crop consumptive use less effective precipitation. In gen- eral, leaching requirements are nominal for the Upper Colorado Region and are not included in this study. Data in Table 8 include the normal range of crop irrigation requirements for the various crops grown in the Upper Colorado Region. Adequacy of supply Shortage of irrigation water is one of the principal problems in the region. It is estimated that about 37 percent of the irrigated land (excluding idle land which receives no irrigation water in an average year) receives inadequate water. Both the quantity and seasonal distri- bution of water affect the adequacy of supply. For example, in areas where water is ample during the spring and early summer, it is a common practice to overirrigate or apply more water than is actually required 27 |