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Relationships defining transmitted (TPAR) and absorbed (APAR) photosynthetically active radiation (PAR) in sorghum [Sorghum bicolor(L.) Moench] canopies do not account for senesced leaves, stalks, and panicles after anthesis. Absorbed PAR by senesced leaves does not directly contribute to photosynthesis, whereas PAR absorbed by panicles does. In addition, senesced leaves, stalks, and panicles reduce transmitted solar radiation reaching the soil and thereby affect soil evaporation. The objective of the study was to examine the combined effect of senesced leaves, stalks, and panicles on TPAR and APAR in a developing sorghum canopy before and after maximum green leaf area index (GLAI) was attained. The study was conducted on a Panoche clay loam (fine‐loamy, mixed, thermic Typic Torriorthent) at the West Side Field Station (CA) near Five Points, CA during 1982 and on a Houston black clay (fine‐montmorillonitic, thermic Udic Pellustert) at the Blackland Research Center (TX) in Temple, TX, during 1983. The experimental designs were three replications of four and five altered canopy treatments (panicles and layers of green and senesced leaves removed) at TX and CA, respectively. Transmitted and absorbed PAR were calculated from PAR measurements above and below the canopy, before and after maximum green leaf area was attained. Transmitted and absorbed PAR differed significantly (95% level) before and after maximum leaf area index was attained. Panicles intercepted 8% of the incoming PAR; stalks 14%; and senesced leaves, 23%. Relationships derived between TPAR and GLAI compared favorably with other published data. Absorption of PAR by panicles, stalks, and senesced leaves should be considered in photosynthesis, evapotranspiration, and crop growth estimates.

Photosynthetically active radiation (PAR) is a necessary input in several crop growth models. Previous research from several locations with different instruments has suggested that PAR could be estimated as a constant fraction of shortwave radiation. This study was conducted to determine if a simple relationship was valid for a large geographic area in the western half of the USA. In the semiarid climate near Fresno, CA, the daily photosynthetic photon flux density (PPFD) in units of μmol m−2was 2.04 ± 0.06 times the solar irradiance (SI). The daily irradiance within the PAR waveband (photosynthetic irradiance (PI), 0.4 to 0.7 μm) was estimated to be 45% of the daily solar irradiance. The diurnal pattern of these relationships was consistent with measurements at Phoenix, AZ and exhibited only a slight diurnal variation. Independent measurements of the solar irradiance between 0.285 and 0.63 μm with “filtered” pyranometers at the Fresno site indicated that irradiance between 0.285 and 0.63 μm was 41% of solar irradiance, a result which was about 7% lower than the estimate from the PAR/SI value converted with a published sun and sky PAR/PI factor. This difference was largely attributed to the differences in measured wavebands. Measurements of solar irradiance less than the waveband of 0.63 μm and total solar irradiance obtained with the same equipment at Brawley, CA; Weslaco, TX; Temple, TX; Manhattan, KS; Lincoln, NE; St. Paul, MN; Fargo, ND; Sidney, MT; Beartooth Pass, WY; Kimberly, ID; Davis, CA were similar to those measured at Fresno, CA. The results indicate that PAR can be estimated from solar irradiance measurements to within 10% (which is probably acceptable for most plant growth models) throughout most of the Western USA.

This irrigation study was undertaken to compare water use efficiencies (WUE) resulting from three methods of timing water applications on narrow‐row cotton (Gossy‐pium hirsutumL.). Irrigation timing during the 1971 and 1972 growing seasons was accomplished using the soil water potential (SWP)‐fixed level, leaf water potential (LWP)‐fixed level, and stress day index (SDI)‐vari‐able level of LWP, based on crop sensitivities, as indicators. Specific levels of each indicator were selected so that nearly identical yields would result for all three timing indicators, thus making possible the valid comparison of WUE. With the SWP indicator as a basis for comparison, higher WUE were obtained with the LWP and SDI indicators in both years. At a plant population of 150,000 plants/ha, average increases in WUE (seed cotton yield divided by irrigation plus rainfall amounts) were 17 and 38% for the LWP and SDI indicators, respectively. Average increases when WUE was calculated as yield divided by irrigation amount only were 30 and 82%, respectively. The increases in WUE were due to reductions in irrigation amounts rather than increases in yield. These results demonstrate that significant benefits in stretching irrigation water supplies are possible when using plant‐based measurements and when plant sensitivities at different growth stages are quantitatively considered in timing irrigations.