**5. Intercepted radiation and leaf area index measurement methods**

#### **5.1 Intercepted radiation measurement**

Quantifying the intercepted radiation (LI) is therefore an important consideration when studying the different agricultural or environmental factors on yield; it is the main source of data in the most widely used methods for estimating crop water needs.

The LI measurement methods are not necessarily destructive, since the provision of plants on the ground plays a key role. However, there are differences between the different methods in terms of the changes introduced into the covers to make measurements, direct methods and indirect methods.

#### **5.1.1 LI measurement with direct methods**

A direct method for determining the percentage of intercepted radiation (LI) is to measure PAR both above and below the canopy at noon on completely cloudless days (Board et al., 1992; Purcell, 2000; Reta-Sánchez y Fowler, 2002):

$$LI = \left| 1 - \left( \frac{PAR \text{ }below \text{ } canopy}{PAR \text{ } above \text{ } canopy} \right) \right| \tag{1}$$

Commercially available lineal PAR sensors are used to take these measurements which are based on PAR values registered by the sensor. These measurements can be taken either by locating sensors perpendicular to the crop rows (Egli, 1994) or by taking multiple measurements parallel to them (Board et al., 1992). The latter method can be costly, according to the number of measurements needed to characterize the study area, especially in the case of low-lying crops, where it may be necessary to remove vegetation in order to place sensors under it, which also has the drawback of introducing alterations during data collection. Using the percentage of shaded soil at solar noon or the percentage of ground cover (PGC) to estimate LI, is an easier and more economical way to obtain the required data. It is generally assumed that the shaded area at soil level corresponds to the fraction of incident radiation which has been intercepted by the crop. This is an approximation that is valid as long as the percentage of light transmission through the leaves is small in comparison to its absorption. The precision with which PGC estimates LI will therefore depend on how well the shaded area is defined and on the capacity of the canopy to capture all of the radiation within the shaded area. In this second case, estimates could be improved by taking complementary measurements of radiation at a sufficient number of points within the shaded area to characterize the radiation traversing the canopy (Lang et al., 1985).

Some of the methods used to determine PGC involve visual estimates (Olmstead et al., 2004; Ortega-Farias et al., 2004). Methods such as the "interception line" (Gallo y Daughtry 1986; Mohillo y Moran, 1991), the analysis the intersection of shadows on metric strips and paper drawings of the sampling areas were used to determine PGC in a non-destructive way (García et al., 2001). However, to apply these last three methods, cloudless days are needed, as a sufficient number of measurements at different orientations are needed to allow a reliable characterization of the area (Ewing y Horton, 1999). The precision of the visual estimation method varies, because it depends on the skill of the operator; results will not be comparable when several people are involved (Olmstead et al., 2004). Furthermore, it has been shown that coverage values tend to be overestimated (Olmstead et al., 2004). In the cases of the interception line and metric strip methods, similar problems are encountered as those associated with the use of PAR bars in the case of low-lying crops and it is difficult to take measurements below the canopy. Finally, making paper drawings is very costly when working under field conditions and when a relatively large area must be characterized.

The LI measurement methods are not necessarily destructive, since the provision of plants on the ground plays a key role. However, there are differences between the different methods in terms of the changes introduced into the covers to make measurements, direct

A direct method for determining the percentage of intercepted radiation (LI) is to measure PAR both above and below the canopy at noon on completely cloudless days (Board et al.,

> <sup>1</sup>

Commercially available lineal PAR sensors are used to take these measurements which are based on PAR values registered by the sensor. These measurements can be taken either by locating sensors perpendicular to the crop rows (Egli, 1994) or by taking multiple measurements parallel to them (Board et al., 1992). The latter method can be costly, according to the number of measurements needed to characterize the study area, especially in the case of low-lying crops, where it may be necessary to remove vegetation in order to place sensors under it, which also has the drawback of introducing alterations during data collection. Using the percentage of shaded soil at solar noon or the percentage of ground cover (PGC) to estimate LI, is an easier and more economical way to obtain the required data. It is generally assumed that the shaded area at soil level corresponds to the fraction of incident radiation which has been intercepted by the crop. This is an approximation that is valid as long as the percentage of light transmission through the leaves is small in comparison to its absorption. The precision with which PGC estimates LI will therefore depend on how well the shaded area is defined and on the capacity of the canopy to capture all of the radiation within the shaded area. In this second case, estimates could be improved by taking complementary measurements of radiation at a sufficient number of points within the shaded area to characterize the radiation traversing

Some of the methods used to determine PGC involve visual estimates (Olmstead et al., 2004; Ortega-Farias et al., 2004). Methods such as the "interception line" (Gallo y Daughtry 1986; Mohillo y Moran, 1991), the analysis the intersection of shadows on metric strips and paper drawings of the sampling areas were used to determine PGC in a non-destructive way (García et al., 2001). However, to apply these last three methods, cloudless days are needed, as a sufficient number of measurements at different orientations are needed to allow a reliable characterization of the area (Ewing y Horton, 1999). The precision of the visual estimation method varies, because it depends on the skill of the operator; results will not be comparable when several people are involved (Olmstead et al., 2004). Furthermore, it has been shown that coverage values tend to be overestimated (Olmstead et al., 2004). In the cases of the interception line and metric strip methods, similar problems are encountered as those associated with the use of PAR bars in the case of low-lying crops and it is difficult to take measurements below the canopy. Finally, making paper drawings is very costly when working under field conditions and when a relatively large area must be characterized.

*PAR above canopy* (1)

*PAR below canopy LI*

methods and indirect methods.

the canopy (Lang et al., 1985).

**5.1.1 LI measurement with direct methods** 

1992; Purcell, 2000; Reta-Sánchez y Fowler, 2002):
