4.7: Intercropping

Some irrigated fields are divided into more than one area for crop rotations. In these cropping systems only one crop is irrigated at a time, so only the ET for that species is relevant for managing that sector. However, interest has grown recently in various forms of intercropping. Intercropping involves growing two or more crops simultaneously juxtaposed within parts of the field. Intercropping includes various forms (Figure 4.16). Mixed intercropping is a complete mixture of multiple species in the same area. Row intercropping involves growing two or more crops at the same time within crop rows. This is common in developing countries and small holdings where an upper story crop—often corn—is first planted and then a shorter crop such as beans is planted in the furrow between crop rows. The crops occupy the same space but may have different growth schedules so the composition of the vegetation changes throughout the season. Alley and strip cropping involves alternating strips of single crops. Strip cropping generally involves paths of equal width off alternating crops. Alley cropping is frequently a form of agroforestry where tree lines are planted beside strips of crops. The width of crop strips in strip and alley cropping is usually some multiple of farming equipment width. Relay and/or cover cropping involves starting a second crop before the first crop is mature or harvested. Crop establishment can be difficult for the second crop in the series.

Computation of water use for intercropped systems is difficult because crops in the mixture have different characteristics. The distribution of leaf area usually involves some shading of lower crops as illustrated for alley cropping in Figure 4.16. Determining the capture of radiation requires information of leaf area distribution vertically and horizontally. Multiple stories of vegetation alter wind patterns within and above crops in the mixture. Rooting characteristics may be quite different leading to dissimilar levels of water stress. The development of the canopy for crops in the mixture may progresses at different rates and the plant density of species in the mixture may vary considerably from field to field. The leaf area, plant geometry and general crop health may be quite different than for single crop fields represented by crop coefficients—especially for small holdings in developing countries. These complexities require more elaborate procedures than simple crop coefficients. Methods are presented by Allen et al. (1998) to estimate compound crop coefficients. Computer models are also available for simulating micrometeorological processes in complex canopies. Methods to estimate intercropped ET are multifaceted and beyond procedures present in this text.

Irrigation of intercropped systems, especially strip and alley cropping, is difficult to achieve efficiently. Water needs of one crop may differ from requirements of the adjacent crop and some irrigation systems are incapable of applying water in that configuration. The soil water along the boundary between crops is often different than in the middle of the strip or alley. This dissimilarity amplifies the distribution of water need within the strip and confounds soil water monitoring. Soil water monitoring can be effective in row intercropping system like shown in Figure 4.16. Landscapes contain a mixture of vegetation that is irrigated at the same time, so the composite ET is needed (Figure 4.16). It is difficult to measure ET for such plantings because of the interactions occurring in the landscape and due to the variability of species in landscapes. Planting densities vary considerably among landscapes. Young landscapes contain less leaf area than mature plantings and are less capable of absorbing radiation; thus, mature landscapes usually have higher transpiration rates. A landscape of trees with underlying shrubs or groundcover captures more radiation and will require more water than trees underlain with mulch. Many landscapes include a range of microclimates varying from shaded or protected areas to hot, sunny, and windy areas. These variations influence ET in ways not representative of large areas of homogeneous vegetation inherent in crop coefficients. Costello and Jones (2014) provide updates to a method to estimate ET using landscape coefficients for multiple species:

\(ET = K_c \times ET_o\) (4.17)

Figure 4.16. Examples of intercropping (upper left photo courtesy of USDA-NRCS).

Table 4.7 Range of plant species, density, and microclimate factors for landscape coefficients (adapted from Costello and Jones, 2014).

Type of Vegetation	Species Factor (KP) High	Species Factor (KP) Avg.	Species Factor (KP) Low	Density Factor (KD) High	Density Factor (KD) Avg.	Density Factor (KD) Low	Microclimate Factor (KM) High	Microclimate Factor (KM) Avg.	Microclimate Factor (KM) Low
Trees	0.9	0.5	0.2	1.3	1.0	0.5	1.4	1.0	0.5
Shrubs	0.7	0.5	0.2	1.1	1.0	0.5	1.3	1.0	0.5
Groundcover	0.7	0.5	0.2	1.1	1.0	0.5	1.2	1.0	0.5
Mixed: Trees, shrubs, and groundcover	0.9	0.5	0.2	1.3	1.1	0.6	1.4	1.0	0.5
Turf grass	0.8	0.7	0.6	1.0	1.0	0.6	1.2	1.0	0.8

where K_L is the landscape coefficient. The amount of ET for a landscape varies as a function of the species planted, the density of vegetation, and microclimate conditions. Assigning numerical values for these factors enables estimation of the landscape coefficient:

K_L = K_P x K_D x K_M (4.18)

(4.18) where K_P is the plant species factor, K_D is the density factor and K_M is the microclimate factor. The range of values for each factor for types of vegetation are presented in Table 4.7. The landscape coefficient procedure differs from the crop coefficient procedure regarding the adequacy of water. Crop coefficients approximate water use for crops under well-watered conditions intended to maximize production. Landscape coefficients approximate the water needed to maintain the aesthetic or functional acceptability of a landscape. Rather than a measure of how much water can be lost from an area, the landscape coefficient is an estimate of the water needed to maintain landscape quality. Species factors (K_p) for five types of vegetation are included in Table 4.7. Three levels are included for each type of vegetation depending on the water use characteristics of the plants included in the landscape. Mixed species plantings have a range of water use like those of tree, shrub, and groundcover species. The values presented in Table 4.7 represent the range assumed for individual species. Costello and Jones (2014) provide species for a very large number of specific plant species to develop an integrated species factor for the landscape. The density of vegetation within a landscape varies considerably. Even though individual plants in a sparsely planted landscape may use more water for a given leaf area than individual plants in a dense landscape, water lost from the entirety of the dense planting will likely be greater than for the sparse landscape. To account for these differences, the density factor varies from a low of 0.5 to a high of 1.3. The density factor involves estimating the percent ground cover for a portion of the landscape. Canopy cover is defined as the percentage of ground shaded. A 50% ground cover will shade half of the land area in the landscape. With a canopy cover less than 60% a reduction in K_D is appropriate. Trees with a canopy cover of 25% or less should have a density factor of 0.5. An upward adjustment of K_D is warranted when trees are the prevailing vegetation, but shrubs and groundcover also occur. Essentially, the groundcover or shrub represents another tier of vegetation where water loss occurs. Total water use would be expected to be greater for multiple tiers than for a single tier. Shrubs and groundcover are equivalent in K_D values. A complete or nearly complete cover (about 90%) with either shrubs or groundcover represents the average condition for these vegetation types and has a density factor 1.0. Higher density values may result when plantings are predominately groundcover or shrubs, but another vegetation type also occurs. Density values for high-density mixed plantings are greater than for other three vegetation types. High density plantings with three vegetation types would be assigned a maximum density factor of 1.3. Low-density mixed plantings may also occur and a commensurate reduction in the density factor is appropriate. Young or widely spaced plantings also qualify for a low-density value.

Figure 4.17. Landscapes with varying plant density and microclimate factors.

Environmental conditions vary significantly within a landscape. Buildings and other structures and paving typical of urban landscapes strongly influence foliar and air temperatures, wind, and humidity. For example, trees in parking lots are subject to higher temperature and lower humidity than trees in parks. Areas within a landscape that have different environmental conditions are called microclimates. Microclimates must be considered in estimating water needs.

The microclimate factor accounts for such differences. The microclimate factors are relatively easy to set. An average microclimate condition is where buildings, pavement, slopes, and reflective surfaces do not influence the microclimate. Essentially, this condition is like that for the reference ET conditions. For these conditions, the microclimate climate factor (K_M) is set to 1.0.

In a “high” microclimate condition, features increase the evaporative condition in the irrigation zone. Landscape surrounded by heat-absorbing surfaces or reflective surfaces or those exposed to particularly windy conditions would be assigned high microclimate factors. For example, medians, parking lots, west sides of buildings, west and south sides of slopes, and wind tunnel areas would be assigned a higher climate factor. Such areas might have a microclimate climate value between 1.0 and 1.4. See Figure 4.17 for examples of high and low microclimate factors.

“Low” microclimate conditions are as common as high microclimate conditions. Plantings that are shaded by buildings or other landscape features for part or most of the day, or that are protected from winds, would be assigned low microclimate values. Examples of conditions that should receive low microclimate factors include areas on the north sides of buildings, courtyards, under wide building overhangs, and the north side of slopes. Such situations would be assigned microclimate values between 0.5 and 1.0 (Figure 4.17).

Application of the landscape methodology is very well developed by Costello and Jones (2014) but is complicated. That publication should be utilized for specific applications. The method may also offer a basis for estimating ET for other intercropped systems.